Preheat oven to 400 degrees F. Heavily oil a 9-inch metal pie pan. Squeeze out excess liquid from grated potatoes and place in large bowl. Mix in salt And egg until well combined. Press potatoes into pie pan and up the sides. If mixture is too sticky, dip fingers in flour. Bake crust for 30 minutes or until it begins to turn golden brown, brushing occasionally with oil.
In a medium skillet melt butter over medium heat and cook onion until tender. Stir in the mushrooms and cook until golden brown.
Spread grated cheese over bottom of potato crust. Spread out the onion and mushroom mixture over the cheese. In a medium bowl, whisk or beat together with a fork the eggs and the cream. Season mixture with salt and pepper and pour over vegetables and cheese.
Reduce oven heat to 375 degrees F and bake for 40 minutes or until firm. Allow quiche to cool for 15 minutes before slicing and serving.