1. Heat the oven to 400°F.
2. Heat the butter in a 10-inch skillet over medium heat. Add the mushrooms, onion and garlic and cook until tender and the liquid is evaporated. Remove the skillet from the heat. Stir in the cheese and basil.
3. Unfold the pastry sheet on a lightly floured surface. Roll the pastry sheet into a 15x12-inch rectangle. Cut into 20 (3-inch) squares. Press the squares into 20 (1 1/2-inch) mini muffin-pan cups. Place about 1 tablespoon filling into each pastry cup.
4. Bake for 15 minutes or until the pastries are golden brown. Let the pastries cool in the pan on a wire rack for 10 minutes.