Canapés can be baked up a day ahead. Cover in plastic wrap and refrigerate. Reheat in preheated oven at 375 F for 10 min. Sprinkle with fresh basil and serve.
Prepare second pastry sheet by doubling ingredients. Otherwise, separate frozen pastry sheets and store leftover sheet in freezer.
Serve as a side dish, by stirring in 2 Tbsp. lemon juice to moisten mushrooms and to deglaze the pan. Add herbs like thyme and sage, if desired.