Cut chicken breast horizontally to make thinner pieces. For chicken thighs, cut the thick parts and fan it out or pound to even out thickness.
In a mixing bowl, make gochujang marinade by adding all sauce ingredients: chopped garlic, grated ginger, soy sauce, gochujang, rice wine (mirin), sugar, sesame oil, Korean red chili pepper, and black pepper. Mix until well blended.
Optionally, cut ½ an onion into thick slices and chop green onions.
Marinate chicken for 2 hours to overnight in the fridge.
Heat oven to 425°F. Line a tray with parchment paper and drizzle vegetable and sesame oil on the tray. Place chicken and onions on the tray.
Bake in the oven for 17-18 minutes. Optionally, set oven to Broil and broil chicken for 1 minute for extra caramelization.
Let it cool for a couple of minutes and sprinkle fresh chopped green onions on top before serving.
