Preheat oven to 400° F.

To make the pastry shells: In a medium mixing bowl, stir together the flour and salt. Using a pastry blender, cut in the butter or margarine until the pieces are the size of small peas. Slowly add the ice water until the mixture is moist enough to form a ball.

On a floured surface, roll the dough to a 1/4-inch thickness. Using a 3-inch-round cookie cutter, cut the dough into circles. Re-roll scraps, if necessary.

Turn a muffin pan upside down - this will become a 'form' to bake the tart shells on. Place the dough rounds over the bottoms (outsides) of 1 3/4-inch muffin cups. Firmly press the dough around the cups. [They will bake this way, and then when removed, they will be in a 'tart' shape and can be filled.]

[Leaving muffin pans upside down] Bake in the 400° oven for 10 to 12 minutes or until the shells are lightly browned. Cool slightly, then remove from the pans.

To make the filling: In a large skillet, melt 2 tablespoons of the butter or margarine. Add the onion and cook until tender but not brown. Add the remaining butter, the mushrooms and lemon juice. Cover and cook for 5 minutes over medium heat. Drain, if necessary; set aside.

In a saucepan, heat the whipping cream until it just starts to boil. Add the salt and pepper. In a small bowl, stir together the cornstarch and water; slowly stir the mixture into the hot cream. Cook and stir until the cornstarch mixture is thickened. Stir in the mushroom mixture. Remove from heat. Spoon the filling into the tart shells and sprinkle with the cheese.

Place the tarts in a shallow baking pan. Broil 4 to 5 inches from the heat for 2 to 3 minutes or until the tarts are puffed and the cheese melts.

If you don't have a pastry blender, you can use 2 table knives to cut in the butter. Hold one knife in each hand and draw the knives across from each other.

If you wish, garnish the tarts with sliced green olives.

Save preparation time by making these appetizers in steps, making the she1ls ahead of time and freezing them. Then, just before serving time, make the filling.