– 1 17.3-ounce package frozen puff pastry (2 sheets), thawed
– 1/4 cup (1/2 stick) unsalted butter
– 12 ounces fresh mushrooms, stemmed, caps cut into 1/4-inch wide strips
– 1 teaspoon coarse kosher salt, divided
– 1/2 teaspoon coarsely ground black pepper, divided
– 1 pound slender asparagus spears, trimmed, cut on diagonal into 1-inch pieces
– 1 1/2 teaspoons chopped fresh thyme
– 1 1/2 teaspoons finely grated lemon peel
– 1/2 cup creme fraiche
– 1/2 cup (packed) coarsely grated Gruyere cheese (about 2 ounces)
– Fresh thyme sprigs (for garnish)
1. Roll out each pastry sheet on work surface to 10-inch square. Cut each into 4 squares
2. Using small knife, score 1/2-inch border (do not cut through pastry) around inside edges of each square
3. Arrange squares on 2 rimmed baking sheets. DO AHEAD: Can be made 1 day ahead. Cover and chill
4. Melt butter in heavy large skillet over medium-high heat. Add mushrooms; sprinkle with 1/4 teaspoon coarse salt and 1/4 teaspoon pepper
5. Saute until tender and lightly browned, about 4 minutes. Transfer mushrooms to large bowl; cool 15 minutes
6. Add asparagus, chopped thyme, lemon peel, salt, and pepper to mushrooms. Mix in creme fraiche and cheese
DO AHEAD: Filling can be made 1 day ahead. Cover; chill.
Position 1 rack in top third and 1 rack in bottom third of oven and preheat to 400 degrees. Mound filling atop pastry squares, leaving 1/2-inch plain border.
Bake tarts 12 minutes. Reverse sheets. Continue to bake tarts until crusts are puffed and golden and filling is cooked through, about 10 minutes longer.
Transfer to plates; garnish with thyme sprigs.
* Sold at most supermarkets and at specialty food stores.