In a large bowl, stir together the pumpkin purée, ricotta, egg, Parmesan, sage, thyme, nutmeg, salt, and pepper until the mixture is smooth and evenly combined.
Melt the butter in a saucepan over medium heat. Stir in the garlic, then whisk in the flour. Slowly pour in the milk while whisking constantly until the sauce becomes silky and thickened. Remove from the heat and stir in the mozzarella until melted.
Spread a thin layer of cream sauce over the bottom of a greased baking dish. Arrange a layer of lasagna noodles, followed by pumpkin filling, more cream sauce, and mozzarella. Continue layering until all of the ingredients have been used, finishing with sauce and cheese on top.
Cover the dish with foil and bake until the noodles are tender and the filling is hot throughout. Remove the foil during the final portion of baking so the cheese develops a beautifully golden finish.
Allow the lasagna to stand for about 15 minutes before slicing. Finish with fresh parsley and serve while warm and creamy.
