Sift flour and salt ina medium bowl. Cut in butter until mizter is crumbly. Sprinkle water over mixture 1 TBLS at a time; mix lightly with a fork just until pastry holds together and leaves side of bowl. Mix lightly to make a ball. Divide into 4 parts. Chill 1 hour.
Roll out each part separtly to a 1/4 inch thickness. Cut into 3 inch rounds and line backs of 2 inch or mini muffin pans. Prick well.
Bake for 10 minutes and cool.
Saute onion in butter until soft. Add mushrooms cooking until liquid evaporates or about 10 minutes. Stir in lemon juice, salt and pepper.
Combine cornstarch with water. Heat cream to just boiling. Remove from heat and stir in cornstarch. Return to heat stiring constantly until mixture thickens.
Stir sauce into muchrooms. Season to taste. Fill baked sheels with 1 TBLS filing each. Sprinkle with cheese. Bake for 10 minutes or until cheese melts.