Tart Shells: Lightly grease 24-30 cups of mini muffin tin. Preheat oven to 400º. Blend butter into flour with fork or pastry blender until it is crumbly. Add the egg and blend together. Add a little cold water at a time until dough is pliable. Cool in refrigerator for 30 minutes. Then roll dough and cut into thin rounds to press into the muffin cups. Prick with fork and bake for 6-8 minutes. Cool before filling.
Tart Filling: Melt butter in large frying pan. Sauté the onion and garlic. Add the mushrooms and sauté an additional 4 minutes. Add the flour and stir another minute. Add the broth and sherry and cook for 1 minute. Then add the cream and cook for an additional minute or two. Add half the bacon. Spoon into prepared pastry tarts. Garnish with remaining bacon. Serve warm.