01 - In a medium bowl, whisk together the soy sauce, mirin, brown sugar, honey, sesame oil, minced garlic, grated ginger, and black pepper until the sugar dissolves. 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, and toss to coat evenly. Refrigerate for 1 to 2 hours.
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.
04 - Thread the marinated steak cubes and vegetables alternately onto skewers, distributing the bell peppers, red onion, pineapple, and mushrooms evenly among them.
05 - Place the kabobs on the preheated grill. Cook for 10 to 12 minutes, turning occasionally and brushing with the reserved marinade, until the steak reaches your desired doneness and the vegetables are lightly charred.
06 - Transfer the kabobs to a serving platter. Sprinkle with toasted sesame seeds and chopped scallions if desired. Serve immediately while hot.