Marinate the Chicken: In a medium bowl, combine the chicken pieces with gochujang, gochugaru, soy sauce, brown sugar, sesame oil, grated ginger, minced garlic, and rice vinegar. Mix well until every piece of chicken is thoroughly coated in the vibrant red marinade. Cover the bowl and let it marinate in the refrigerator for at least 20 minutes, or up to 2 hours for deeper flavour.

Prepare the Sauce Base: While the chicken marinates, whisk together the chicken stock with an additional teaspoon of gochujang and a pinch of brown sugar in a small bowl. This liquid will form the rich, glossy sauce later on.

Sauté the Onion: Heat 1 tablespoon of vegetable oil in a large frying pan or skillet over medium-high heat. Add the thinly sliced onion and sauté, stirring occasionally, until it turns translucent and starts to soften, releasing a sweet aroma, about 5-7 minutes. The edges should just begin to turn a light golden-brown.

Cook the Chicken: Add the marinated chicken to the pan with the softened onions. Spread the chicken out in a single layer as much as possible. Cook for 8-10 minutes, stirring occasionally, until the chicken pieces are no longer pink on the outside and have developed a lovely caramelised crust, smelling intensely savoury and spicy.

Simmer in Sauce: Pour the prepared chicken stock mixture over the chicken. Bring the liquid to a gentle simmer, then reduce the heat to medium-low. Let it bubble gently, stirring occasionally, for 10-12 minutes. The sauce will begin to thicken and turn a deep, glossy crimson, clinging beautifully to the chicken, and the entire kitchen will be filled with a tantalising spicy aroma.

Add the Cheese and Serve: Once the sauce has thickened to your liking and the chicken is fully cooked through, sprinkle the grated mozzarella and cheddar cheeses evenly over the top of the chicken. Cover the pan with a lid or foil and let it sit for 3-4 minutes, or until the cheese is completely melted, bubbly, and beautifully stretchy. Garnish generously with chopped spring onions and a sprinkle of sesame seeds before serving immediately.