01 - Preheat the oven to 400°F.
02 - Place potatoes in a large pot, cover with cold water, and add a pinch of salt. Bring to a boil and simmer for 15 to 18 minutes until fork-tender.
03 - While potatoes cook, heat olive oil in a large skillet over medium heat. Add onion, garlic, carrots, and celery. Sauté for 5 minutes until softened.
04 - Add ground beef to the skillet. Cook, breaking it up, until browned and no longer pink, about 6 to 8 minutes. Drain excess fat if needed.
05 - Stir in tomato paste, Worcestershire sauce, thyme, rosemary, salt, and pepper. Cook for 2 minutes.
06 - Add beef broth and bring to a simmer. Stir in frozen peas. Cook for 5 minutes, letting the mixture thicken slightly. Remove from heat.
07 - Drain the potatoes. Mash with milk, butter, salt, and white pepper until smooth and creamy.
08 - Spread the beef filling evenly in a 2-quart baking dish. Top with the mashed potatoes, spreading to cover completely. Use a fork to create ridges for extra crispiness.
09 - Bake for 25 minutes or until the top is golden brown and the filling is bubbling. Let stand for 10 minutes before serving.