Heat sesame oil in a large skillet over medium heat. Add sliced onion and minced garlic; sauté for 3–4 minutes until softened.
Stir in gochujang, tamari (or soy sauce), sugar, and gochugaru. Cook for 1–2 minutes until the mixture is well combined.
Pour in the milk, stirring continuously to create a smooth, creamy sauce.
Increase heat to medium-high. Add the rice cakes, fish cakes, and mini sausages. Once the sauce begins to boil, reduce heat to a simmer. Cook for 4–5 minutes, stirring occasionally, until rice cakes are tender.
Lower the heat and mix in shredded mozzarella and grated parmesan cheese. Stir until the cheese melts and the sauce becomes thick and creamy.
Remove from heat. Garnish with sliced green onions and sesame seeds. Serve hot.