Preheat oven to 375° F.
Beat the eggs, cream, grated cheese, and pepper until thoroughly mixed.
Cook the onions in butter in fry pan until they are just beginning to turn translucent. Add mushrooms and lemon juice. Cook over moderate heat until mushrooms give up their liquid. Then turn up heat and boil until liquid is completely evaporated and mushrooms are beginning to sauté in their butter. Turn off heat and let rest off burner for a minute or so until mushrooms cool a bit.
Spoon mushrooms and their butter into pastry shells. Pour cream, egg and cheese mixture over mushrooms until shells are filled. Place shells (on the baking sheet*) into the oven and bake 25± minutes until top is a light brown and quiche is a bit puffy. Do not overcook or quiche will be unpleasantly dry.
Remove from oven and let rest 5-10 minutes.