Preheat oven to 400F and butter a large casserole dish.
In a large pot over medium high heat, sauté the onion, garlic, and mushrooms until the onions are translucent and the mushrooms lose some of their water.
Add butter and flour to coat the vegetables and cook for 3 minutes. Add milk and whisk to avoid lumps forming from the flour. Season by adding your hot sauce or Gochujang, as well as salt to taste.
Stir in shredded cheese and dump in cooked pasta. Coat pasta and place in casserole dish.
Top by mixing panko with crushed dried kimchi and bake for 15-25 minutes.
Notes
If you can't find Korean hot pepper paste, substitute for the ever so popular Sriracha!