In a deep frying pan, heat about 1 tbsp oil over medium heat. Transfer the marinated chicken thighs to the pan, and sear both sides until 70-80% cooked (about 2-3 minutes). Set aside.
Using the same pan, heat another tbsp oil over medium heat. Add in sliced onions and minced garlic, then stir-fry till fragrant (about 1-2 minutes).
Add in leeks and sausages, and stir-fry until slightly golden brown (about 1 minute). Add in Korean rice cakes (see note 1) and give it a rough stir.
Add in tteokbokki sauce and heavy cream, then mix until the rosé sauce is well combined.
When the rice cakes become slightly tender, transfer the chicken thighs back to the frying pan, and stir to mix well (about 3-5 minutes).
Add in chopped spring onions, give it a quick stir, and add in mozzarella cheese.
Cover with a lid, turn down the heat to low, and simmer for another 2-3 minutes until the cheese has melted.
Transfer the dish to a serving plate, and garnish with extra chopped spring onions.
Enjoy the dish fresh and warm!