Begin your culinary journey by marinating the diced chicken breasts in 2 tablespoons of Japanese BBQ sauce for at least 15 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet over medium-high heat and cook the marinated chicken for about 5 to 7 minutes.
Add another splash of vegetable oil, crack in the large eggs, and scramble until softly set, about 2 minutes.
Sauté the minced garlic and freshly grated ginger in the same skillet for about 30 seconds, then add the medium onion and mixed vegetables, cooking for an additional 3 to 4 minutes.
Add the day-old cooked rice, drizzle with low-sodium soy sauce and toasted sesame oil, and stir-fry for about 3 to 5 minutes until heated through.
Return the cooked chicken and scrambled eggs to the skillet, add the remaining Japanese BBQ sauce, and sauté for an additional 2 minutes.
Season with salt and freshly ground black pepper, garnish with sliced green onions, and serve hot.