How to Make It
Preheat oven to 450ºF. Melt 1 Tbsp. butter in a large nonstick skillet over medium-high heat. Add mushrooms and cook, stirring often, until golden, about 8 minutes. Add garlic and thyme; sauté 1 minute longer. Season with salt and pepper. Add sherry; cook until almost evaporated, about 30 seconds. Stir in spinach. Remove from heat.
Melt remaining 1 Tbsp. butter in a small heavy saucepan over medium heat. Whisk in flour to form a paste and continue to cook, whisking, for 1 minute. Gradually add milk, whisking constantly, and bring to a simmer. Cook until sauce has slightly thickened, about 7 minutes. Remove from heat; season with salt, pepper and nutmeg. Whisk in half of Gruyère.
Stir 1/2 cup of sauce into mushroom mixture. Place 1 crepe on a work surface; arrange 1/4 of filling (a scant 1/3 cup) over half of crepe. Fold unfilled side over filled side; fold in half again (it will look like a triangle). Repeat with remaining crepes and filling.
Coat a shallow 7-by-11-inch baking dish with cooking spray. Arrange crepes in baking dish, overlapping slightly. Cover crepes with remaining sauce. Sprinkle with remaining Gruyère. Bake on top rack until bubbling and golden, about 12 minutes. Let stand for 5 minutes before serving.