Preheat oven to 400°. Peel pumpkin; cut in half lengthwise; discard seeds or save for toasting. Cut into 1/4-in. thick slices. Place in a single layer on two greased 15x10x1-in. baking pan. Drizzle with oil; sprinkle with 1/4 teaspoon salt nutmeg. Roast until tender, 30-35 minutes. Reduce oven temperature to 350°.
Meanwhile, cook lasagna noodles according to package directions for al dente. In a large saucepan, melt butter over medium heat. Add onion; cook and stir until tender, 6-7 minutes. Add garlic; cook 1 minute longer. Stir in flour and remaining 3/4 teaspoon salt until smooth; gradually whisk in milk and sage. Bring to a boil, stirring constantly; cook and stir until thickened, 8-10 minutes. Remove from the heat; stir in Parmesan cheese. Drain noodles.
Place 3 noodles in a greased 13×9-in. baking dish. Layer with 1/3 of the pumpkin, 1-1/3 cups sauce and 1/2 cup mozzarella cheese. Repeat layers twice. Top with remaining noodles, sauce and mozzarella cheese.
Bake, covered, 30 minutes. Uncover and bake until golden brown bubbly 30-35 minutes longer. Let stand 10-15 minutes before serving. If desired, sprinkle with red pepper flakes.
