Marinate the chicken: Chop the chicken breast into bite-size pieces. Mix the chicken breasts with cornstarch, sugar, Shaoxing wine, soy sauce, and a pinch of salt. Marinate for 10-15 minutes. Meanwhile, prepare other ingredients.
Prepare other ingredients: Mince the garlic clove, dice the onion, and make a few cuts to the fermented kimchi. Set aside.
Cook the chicken: In a frying pan, heat some oil over medium heat, and sear both sides of the chicken breasts until golden brown. When they are just cooked, remove from heat, and set aside.
Sauté aromatics: Using the same pan, sauté diced onions and minced garlic with the remaining oil from the chicken until the onions become translucent. Add in chopped kimchi with any kimchi sauce, and stir-fry until fragrant.
Fry the rice: Add in day-old rice, gochujang, gochugaru, soy sauce, dark soy sauce, and sugar. Stir-fry until well combined, breaking up the cold rice until all rice grains are coated with seasonings.
Place the cooked chicken back in the pan: Transfer the cooked chicken back to the pan, and stir-fry together. Season with black pepper and mix well.
Serve: Transfer the kimchi fried rice to a bowl or plate, top with a fried egg, and garnish with toasted sesame seeds, and chopped spring onions. Enjoy!