Prepare the Crepe Batter: In a large mixing bowl, whisk together 1 cup of flour, 2 eggs, 1 cup of milk, and a pinch of salt until just combined. Allow the batter to rest for at least 30 minutes at room temperature.

Cook the Crepes: Heat a lightly greased non-stick skillet over medium heat. Pour about 1/4 cup of the crepe batter into the center and quickly swirl. Cook for about 1-2 minutes or until golden, flipping to cook the other side.

Sauté Filling: In a separate skillet, melt 2 tablespoons of butter. Add 1 chopped leek and 8 ounces of sliced mushrooms; sauté until tender, then add 2 cups of spinach and the shredded chicken until heated through.

Make Béchamel Sauce: In a saucepan, melt 3 tablespoons of unsalted butter, whisk in 3 tablespoons of flour. Gradually add in 2 cups of warm milk, stirring until thickened. Season with salt and nutmeg.

Assemble Crepes: Preheat oven to 350°F. Fill each crepe with chicken mixture, roll, and place seam-side down in a greased dish.

Top and Bake: Pour béchamel sauce over crepes and sprinkle 1 cup of Gruyere cheese on top. Bake for 25-30 minutes until bubbly and golden.