Begin by cubing the boneless skinless chicken breasts into 1-2 inch pieces. In a bowl, mix together the soy sauce, hoisin sauce, rice wine vinegar, minced garlic, grated ginger, and toasted sesame oil. Toss the chicken in this marinade, ensuring it's well-coated, and let it sit for at least 20 minutes or up to 4 hours in the fridge.

While the chicken marinates, heat a cast iron skillet or flat-top griddle over high heat. Add about 2 tablespoons of peanut oil and allow it to heat until shimmering, about 3-5 minutes.

Add the marinated chicken pieces in a single layer and let them cook undisturbed for 2-3 minutes until golden brown. Stir and continue cooking for another 3-4 minutes until the chicken reaches an internal temperature of 165°F.

In the same pan, add the mixed vegetables and stir-fry for about 4-5 minutes or until they are tender yet retain some crunch.

Move the vegetables to one side and crack an egg onto the other side. Scramble the egg quickly, incorporating minced garlic and diced onions. Once fully cooked, stir in the leftover rice and mix everything together, letting it fry undisturbed for the first 3 minutes.

Plate the sautéed chicken and vegetables alongside the fried rice. Drizzle some soy sauce over the top if desired and garnish with sesame seeds or chopped green onions.