Asparagus And Mushroom Tarts

Name: Asparagus And Mushroom Tarts
Ingredients: 1 17.3-ounce package frozen puff pastry (2 sheets), thawed; 1/4 cup (1/2 stick) unsalted butter; 12 ounces fresh shiitake 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 crème fraîche; 1/2 cup (packed) coarsely grated Gruyère cheese (about 2 ounces); Fresh thyme sprigs (for garnish)