In a large mixing bowl, combine the gochugaru, gochujang, soy sauce, vegetable oil, sesame oil, rice syrup, chopped chilies, garlic, ginger, salt, and black pepper. Stir until a thick, vibrant red paste forms.
Add the chopped chicken thighs to the bowl and mix thoroughly, ensuring every piece is fully coated in the sauce. Cover the bowl and refrigerate for at least 1 hour.
Heat a large oven-proof skillet over medium heat. Add the marinated chicken along with the water or chicken stock. Cover the pan and let the chicken simmer for about 10 minutes, stirring occasionally.
Remove the lid and continue cooking for a few minutes until the sauce becomes glossy and clings tightly to the chicken.
Set your oven to BROIL. Sprinkle the shredded mozzarella evenly over the chicken. Place the pan under the broiler for 2–3 minutes, just until the cheese melts, bubbles, and begins to lightly brown.
Remove from the oven and garnish with chopped green onions or parsley, plus extra pepper flakes if you like more heat. Serve immediately while the cheese is hot and stretchy.
