Melt the margarine in a small pan. Add onions, and saute over medium heat for a few minutes. When they begin to soften, add mushrooms, salt, pepper, thyme, and mustard. Saute about 5 minutes more and remove from heat.
Combine eggs, milk and flour and beat well.
Spread the grated cheese over the bottom of the unbaked crust, and spread the onion-mushroom mixture on top. Pour in the egg mixture and garnish with paprika.
Bake for 35-45 minutes, or until solid in the center. Serve hot.