In a bowl, combine minced garlic, soy sauce, sake, sugar, mirin, sesame oil, grated ginger, and black pepper to make the yakiniku marinade.
Place the thinly sliced beef into a resealable plastic bag or a shallow dish. Pour the marinade over the beef, ensuring it's evenly coated. Seal the bag or cover the dish and let it marinate in the refrigerator for at least 15 minutes.
Heat a grill pan or a non-stick skillet over high heat. When it's hot, add the marinated beef slices in a single layer. Grill for about 1-2 minutes on each side until they're cooked to your desired level of doneness. The beef cooks quickly due to its thinness.
While grilling the beef, heat the leftover marinade in a small saucepan and bring it to a boil. Simmer for a few minutes until it thickens slightly, then remove from heat.
To assemble the Yakiniku Donburi, place a serving of cooked rice in each bowl or on a plate.
Arrange the grilled beef slices on top of the rice.
Steam or quickly saut your choice of mixed greens or vegetables until they're tender-crisp, then place them alongside the beef.
Drizzle the thickened marinade sauce over the beef and vegetables.
Garnish with sesame seeds and sliced green onions.
Serve the Yakiniku Donburi hot and enjoy your homemade Japanese barbecue experience!
