Preheat oven to 400 degrees
Roll out each pastry sheet to 10-inch square. Cut each sheet into 4 squares, then cut each of those squares into 4 squares- so 32 squares in all. Place squares in mini-muffin cups coated with cooking spray.
Melt butter in large skillet over medium heat. Add mushrooms; sprinkle with 1/4 teaspoon kosher salt and 1/4 teaspoon pepper. Sauté until tender and lightly browned, about 4 minutes. Transfer to large bowl; cool 15 minutes. Add asparagus, thyme, zested lemon peel, 3/4 teaspoon kosher salt, and 1/4 teaspoon pepper to mushroom mixture. Mix in crème fraîche and cheese.
Mound filling in prepared muffin cups.
Bake cups until crusts are puffed and golden and filling is cooked through, about 18 minutes. Serve warm.