01 - In a medium bowl, whisk together soy sauce, mirin, brown sugar, honey, sesame oil, garlic, ginger, and black pepper until fully combined. Reserve ¼ cup of the marinade in a separate container for basting during grilling.
02 - Place the sirloin cubes in a large resealable plastic bag or shallow dish. Pour the remaining marinade over the steak, seal tightly, and toss to coat evenly. Refrigerate for 1 to 2 hours to allow the flavors to penetrate the meat.
03 - Preheat the grill to medium-high heat, approximately 400°F. If using wooden skewers, submerge them in water for at least 30 minutes to prevent burning during grilling.
04 - Thread the marinated steak cubes and prepared vegetables onto the skewers in an alternating pattern, distributing the bell peppers, red onion, pineapple chunks, and mushrooms evenly across each skewer.
05 - Place the assembled kabobs on the preheated grill. Cook for 10 to 12 minutes, turning every 2 to 3 minutes and basting generously with the reserved marinade, until the steak reaches your preferred doneness and the vegetables develop light char marks.
06 - Transfer the grilled kabobs to a serving platter. Sprinkle with toasted sesame seeds and chopped scallions. Serve immediately while hot.