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 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, approximately 400°F. If using wooden skewers, submerge them in water for at least 30 minutes to prevent burning.
04 - Remove the steak from the marinade, allowing excess to drip off. Thread the steak cubes, bell peppers, red onion, pineapple, and mushrooms alternately onto the skewers, distributing ingredients evenly.
05 - Place the skewers on the preheated grill. Cook for 10 to 12 minutes, turning every 2 to 3 minutes and brushing generously with the reserved marinade, until the steak reaches your desired doneness and the vegetables show light charring.
06 - Transfer the grilled kabobs to a serving platter. Sprinkle with toasted sesame seeds and chopped scallions. Serve immediately while hot.