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 bag or shallow dish. Pour the remaining marinade over the steak, seal tightly, and toss to coat evenly. Refrigerate for 1 to 2 hours.
03 - Preheat your grill to medium-high heat, approximately 400°F. If using wooden skewers, submerge them in water for at least 30 minutes to prevent burning.
04 - Thread the marinated steak cubes onto skewers, alternating with red bell pepper, yellow bell pepper, red onion, pineapple chunks, and mushrooms. Distribute ingredients evenly across all skewers.
05 - Place the assembled skewers on the preheated grill. Cook for 10 to 12 minutes, turning every 2 to 3 minutes and basting with the reserved marinade. Cook until the steak reaches your preferred doneness and the vegetables develop a light char.
06 - Transfer the grilled kabobs to a serving platter. Garnish with toasted sesame seeds and chopped scallions if desired. Serve immediately while hot.