Preheat oven to 350°. Place pecans in a single layer on a baking sheet, and bake, stirring once, 7 minutes or until toasted. Transfer to a plate to cool; chop.
Melt 2 tablespoons butter in a large nonstick skillet over medium heat. Add pumpkin, onion, garlic, and 3/4 teaspoon salt; cook, stirring often, 15 minutes or until pumpkin is just tender. Stir in pecans; set aside.
Cook lasagna noodles according to package directions. Rinse with cool water, and cover with plastic wrap until ready to assemble.
Melt remaining 3 tablespoons butter in a medium saucepan over medium heat. Add flour and cook, whisking constantly, 1 minute. Gradually whisk in milk. Add sage, pepper, and remaining 3/4 teaspoon salt; cook, whisking constantly, about 9 minutes or until thickened. Remove from heat, and whisk in 1 cup Parmesan cheese.
Spread 1/3 cup sauce in a greased 13- x 9-inch baking dish. Top with 3 lasagna noodles. Spread with about 2/3 cup sauce, half pumpkin mixture (about 2 1/4 cups), and 3/4 cup mozzarella. Top with 3 more lasagna noodles. Spread 2/3 cup sauce over noodles, top with remaining pumpkin mixture, and sprinkle with mozzarella. Top with remaining 3 lasagna noodles. Spread remaining sauce over noodles. Sprinkle with 1/4 cup Parmesan cheese. Cover tightly with aluminum foil, and bake 30 minutes.
Remove foil; sprinkle with remaining 1/4 cup Parmesan, and bake 10 minutes or until golden brown. Let stand 10 minutes before serving.