Heat oven to 375°. Spray 24 miniature muffin cups with PAM. Clean mushrooms and finely chop. In small skillet, combine margarine, garlic and mushrooms. Cook and stir over medium heat for 5 to 6 minutes or until margarine is absorbed and mushrooms are tender. Stir in onions and mustard. Unroll dough into 4 rectangles. Seal perforations. Cut each rectangle into 6 squares. Press 1 square into each muffin cup, letting corners stand above top of cups. Spoon a tablespoon of mushroom mixture into each cup. Bake for 10 to 12 minutes or until light golden brown. Meanwhile, cut Brie into 24 pieces. Remove cups from oven. Place Brie in each cup. Bake 2 to 4 minutes until cups are golden brown and cheese is softened. Cool 5 minutes. Remove from muffin cups.