Bring a pot of water to a boil. Once boiling, drop in the frozen udon and let it blanch for about a minute or until the noodles begin to loosen up and separate. Reserve some of the boiling liquid, then drain. Set the udon aside until use.

Melt the butter in a pan over medium heat. Add the sliced mushrooms and sauté until the mushrooms have softened, browned, and their juices begin to release, about 3-4 minutes.

Add the minced garlic and continue to sauté until the garlic is golden, about 1 minute.

Add the heavy cream, reserved udon boiling liquid, gochujang, soy sauce, and sugar. Mix thoroughly until smooth and no clumps of paste are left. Simmer until slightly thickened, about 1 minute.

Add the cooked udon. Drizzle in the sesame oil. Mix to combine and continue to cook for an additional 1-2 minutes until the sauce coats the noodles, stirring occasionally.

Top with chopped scallions and sesame seeds.