Roast butternut squash. Peel & mash until smooth.
Preheat to 350.
Cook lasagna noodles al dente. Drain, then lay flat on a clean kitchen towel.
Cream together ricotta, parmesan, egg, sage, salt, & pepper. Fold in mashed butternut squash.
Spread 3-4 tablespoons of ricotta mixture on each lasagna noodles. Redistribute filling as needed so they're all evenly covered, then sprinkle with 1 1/2 cups mozzarella.
Tightly roll up each lasagna noodle.
Heat butter in a large skillet over medium heat until sizzling. Whisk in flour and let cook for 2-3 minutes, until lightly golden.
Slowly whisk milk into the flour mixture, then add nutmeg, salt, & pepper. Continue whisking for 3-4 minutes until slightly thickened, then remove from heat.
Spread 1/2 cup sauce over the bottom of a 9x13 baking dish. Place rolled lasagna seam-down in the dish, then cover evenly with remaining sauce. Sprinkle with remaining 1/2 cup mozzarella.
Bake at 350 for 25 to 30 minutes, until golden and bubbling.