01 - In a medium bowl, whisk together the soy sauce, mirin, brown sugar, honey, sesame oil, minced garlic, grated ginger, and black pepper until fully combined. Reserve ¼ cup of the marinade in a separate container for basting later.
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.
03 - Preheat the grill to medium-high heat. If using wooden skewers, submerge them in water for at least 30 minutes to prevent burning during grilling.
04 - Thread the marinated steak cubes onto skewers, alternating with red bell pepper, yellow bell pepper, red onion, pineapple chunks, and mushrooms. Distribute the ingredients evenly across each skewer.
05 - Place the assembled kabobs on the preheated grill. Cook for 10 to 12 minutes, turning occasionally and brushing generously with the reserved marinade, until the steak reaches your desired doneness and the vegetables are lightly charred.
06 - Transfer the grilled kabobs to a serving platter. Sprinkle with toasted sesame seeds and chopped scallions. Serve immediately while hot.