In a medium skillet on low to medium heat, sauté sliced mushrooms in butter until soft. Add diced onions, cook until translucent. Add the zucchini, cook until slightly softened.
In a small bowl, blend the sour cream, flour, salt, garlic powder, paprika, and pepper. Stir into the mushrooms and heat to boiling then remove from the heat immediately. Pour into a small ungreased casserole dish.
Combine the parsley and shredded Gruyère and Swiss cheese, sprinkle on top of the casserole. Bake uncovered for 10 to 15 minutes, or until the cheese is bubbling. Enjoy!