In a large bowl, place the gochujang, chilli flakes, sweet paprika, soy sauce, 1 tablespoon of the vegetable oil, pepper, sugar and garlic in a bowl. Mix until well combined.
Add the chicken and mix until evenly coated.
Preheat the oven grill or broiler on high.
In a large oven-proof frying pan or casserole dish, heat the remaining 1 tablespoon of oil over medium-high heat. Add the chicken mixture. Stir-fry for a couple of minutes. Then add a ¼ cup of water and stir to combine. Cover with a lid and turn the heat to low. Cook for about 10 minutes, stirring every so often, until the chicken is just cooked. Remove the lid and top the chicken with the mozzarella. Place under the oven grill/broiler and cook until cheese is melted and golden. Sprinkle with spring onion and serve.