In a large pot of salted water boil egg noodles until al dente. Drain well.
In a large saucepan over medium-high heat saute butter, onion, and garlic until onions are soft. Add flour and mushrooms. Cook 5 minutes.
Add mushroom soup to saucepan, simmer uncovered 10 minutes.
Add cooked halibut and stir in sour cream. Heat through. Season with salt and pepper. Serve sauce over egg noodles, sprinkle parsley on top. Serve hot.