Toss butternut squash with olive oil, 1 tablespoon garlic, ground sage, and seasoned salt in a 9x11-inch baking dish until coated.
Roast in the preheated oven, stirring every 20 minutes, until butternut squash is tender, about 1 hour.
Reduce oven temperature to 375 degrees F (190 degrees C).
Melt butter in a large saucepan over medium heat. Add garlic; cook and stir until fragrant, about 1 minute. Whisk in flour until a smooth paste forms. Slowly pour in milk; season with salt and pepper. Cook and stir sauce until thick and bubbling, 8 to 10 minutes.
Remove sauce from heat; stir in roasted butternut squash, chili powder, ground sage, and rosemary. Puree sauce with an immersion blender until smooth.
Mix ricotta cheese, 1 1/2 cup mozzarella cheese, spinach, egg, and Parmesan cheese together in a bowl.
Spread 1/2 cup sauce in the bottom of the baking dish. Lay 3 lasagna noodles on top. Spread 1/3 of the ricotta cheese mixture over the noodles. Repeat layers twice more, ending with remaining sauce. Sprinkle remaining 1/2 cup mozzarella cheese on top. Cover baking dish with aluminum foil.
Bake in the preheated oven until mozzarella cheese is melted, about 40 minutes. Uncover and continue baking until bubbly, about 10 minutes more.