Savory Mushroom Onion Tart

There's a particular magic to watching onions transform in a hot pan—that moment when they shift from sharp and peppery to glossy and sweet, almost caramelized. I stumbled upon this tart while trying to use up a pile of mushrooms from the farmer's market and some onions that had been sitting in my pantry longer than I'd like to admit. The result was so elegantly simple and impressively golden that I've been making it ever since, sometimes for dinner parties where I want to look like I spent all day cooking, and sometimes just for myself on a quiet evening.

My neighbor knocked on my door one Saturday afternoon when this tart was cooling on the rack, and the smell alone drew her inside—she ended up staying for a slice and a glass of wine, and we spent the next two hours talking about food and family and life. That's when I realized this recipe isn't just about feeding people; it's about creating those unexpected, unhurried moments that stick with you.

Ingredients

• Ready-made puff pastry (1 sheet, about 250 g): Store-bought is absolutely fine and saves you from making it from scratch—there's no shame in smart shortcuts.
• Olive oil (2 tablespoons): Good quality makes a subtle but real difference in how the onions taste.
• Unsalted butter (1 tablespoon): This combination with olive oil gives you better browning and deeper flavor than either alone.
• Yellow onions (2 large, thinly sliced): The thinner you slice them, the more evenly they caramelize and the faster they cook.
• Cremini or button mushrooms (400 g/14 oz, sliced): Cremini are earthier if you can find them, but button mushrooms work beautifully and cost less.
• Garlic (2 cloves, minced): Add it at the end so it perfumes the vegetables without turning bitter.
• Eggs (3 large): They're what binds the filling and gives it that silky, custard-like texture.
• Heavy cream (150 ml/2/3 cup): This is what makes the filling luxurious—don't skip it or substitute with milk.
• Gruyère or Swiss cheese (100 g/1 cup, grated): Gruyère has a nuttier, deeper flavor that pairs perfectly with mushrooms.
• Fresh thyme (1 tablespoon or 1 teaspoon dried): Fresh is noticeably better here, but dried works if that's what you have.
• Salt and black pepper: Season to taste—mushrooms absorb a lot of salt, so be generous.
• Fresh chives or parsley (optional garnish): A handful scattered on top just before serving brightens everything.

Instructions

Set your oven and prepare:

Heat your oven to 200°C (400°F) and line a 23 cm tart pan with parchment paper. This prevents sticking and makes cleanup easier than you'd think.

Settle the pastry:

Roll out your thawed puff pastry and press it gently into the pan, then trim any overhang with a knife. Prick the base all over with a fork to prevent puffing, then slide it into the fridge while you work on the filling—cold pastry bakes up flakier.

Begin the caramelization:

Pour olive oil and butter into a large skillet over medium heat and let them warm for a minute. Add your sliced onions and stir them every few minutes, letting them soften and turn golden around the edges, which takes about 12 minutes—don't rush this step because that's where the sweetness comes from.

Add the mushrooms:

Once the onions look glossy and soft, add your sliced mushrooms and keep stirring. They'll release moisture at first, then that liquid will evaporate and they'll turn golden and concentrate in flavor, taking another 8–10 minutes. This is when your kitchen will smell absolutely incredible.

Finish the vegetables:

Stir in your minced garlic and thyme for just about a minute, until fragrant, then take the pan off heat and let everything cool slightly. You want it warm, not piping hot, before you add it to the pastry.

Make the custard:

In a bowl, whisk together your eggs, heavy cream, salt, and pepper until well combined. Stir in your grated cheese and mix until smooth.

Assemble and bake:

Spread the cooled onion and mushroom mixture across your chilled pastry shell, then pour the egg and cheese mixture evenly over top. Slide it into the preheated oven and bake for 25–30 minutes until the filling is set (it should wobble just slightly in the very center) and the pastry is golden brown.

Cool and serve:

Let the tart rest for about 10 minutes so the filling sets fully and becomes easier to slice. Scatter fresh herbs on top if you're using them, then slice and serve warm or at room temperature.

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I remember bringing this tart to a potluck where I was nervous about whether it would hold up during transport, and it arrived perfect—golden and fragrant and somehow more elegant than everything else on the table. Watching people come back for seconds and ask for the recipe taught me that food you make with care has a way of telling that story on its own.

The Science of Caramelization

Caramelizing onions isn't actually caramelizing in the technical sense—it's the Maillard reaction, where heat turns the natural sugars and proteins into hundreds of new compounds that taste deep and complex. This takes patience and medium heat; high heat will burn them instead of sweetening them. The reward for waiting is a flavor that tastes like it took hours but only requires about 12 minutes of actual attention.

Making It Your Own

This tart is honestly a vehicle for whatever vegetables and cheeses you love. I've made it with leeks instead of onions, roasted red peppers alongside the mushrooms, and even a handful of spinach stirred in. The only real rule is that whatever you add should be cooked and cooled before it goes into the custard, otherwise you'll end up with excess moisture that makes the filling weep.

Serving and Storage

This tart is at home as a main course with a crisp green salad and a cold glass of white wine, or cut into smaller pieces as an elegant appetizer. It keeps well in the refrigerator for up to three days and tastes fine at room temperature, making it perfect for make-ahead meals.

• For a splash of extra depth, add a small pour of dry white wine when you're cooking the mushrooms and let it simmer away almost completely.
• If you can't find Gruyère, good aged cheddar or even a crumbly goat cheese will shift the flavor in delicious directions.
• Serve with a simple arugula salad dressed with lemon vinaigrette to cut through the richness.

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This tart has become one of those recipes I reach for when I want to feel grounded in the kitchen, when cooking feels less like checking boxes and more like a small act of care. There's something deeply satisfying about turning a few simple vegetables into something that looks and tastes like you spent your whole day on it.

Recipe FAQs

→ How do I get the onions perfectly caramelized?

Cook sliced onions over medium heat with olive oil and butter, stirring frequently, until they turn soft and golden—about 12 minutes ensures a deep caramelization.

→ Can I substitute the Gruyère cheese?

Yes, goat cheese or feta work well to add different but complementary flavors to the tart.

→ What type of mushrooms work best?

Cremini or button mushrooms are ideal for their earthy flavor and texture when cooked down.

→ Is it necessary to chill the pastry before adding filling?

Chilling the pastry helps maintain its shape and prevents shrinking during baking, resulting in a flaky crust.

→ How can I enhance the mushroom flavor?

Adding a splash of white wine while cooking mushrooms deepens their flavor and adds complexity.

→ What herbs complement this tart best?

Fresh thyme in the filling and chives or parsley as garnish bring aromatic freshness and a vibrant finish.