Preheat oven to 375F.
Melt the butter 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 in a blender or food processor, 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 custard, and sprinkle top with paprika.
Bake for 35-45 minutes, or until solid in the center. Serve hot, warm, or at room temperature. --