If you can find them, use fresh sheets of pasta in place of the oven-ready noodles. Simply lay the fresh sheets right in the pan.
1 medium butternut squash (about 2 pounds) peeled and cut into 1-inch cubes (about 6 cups)
2 tablespoons olive oil
Salt and ground black pepper
6 to 8 ripple-style oven-ready lasagna noodles, such as Ronzoni
1 container (7 to 8 ounces) crème fraiche or sour cream
1/2 cup grated Parmesan cheese (about 2 ounces)
1 container (about 8 ounces) ricotta cheese
1/4 cup minced fresh sage leaves, plus 8 whole leaves
Adjust oven rack to middle position and heat oven to 400 degrees.
Toss squash with olive oil and a light sprinkling of salt and pepper; turn into a large rimmed baking sheet in a single layer and roast until tender and lightly brown, about 30 minutes. Set aside to cool slightly.
Meanwhile, mix 1 1/2 tablespoons salt and 2 quarts very hot tap water in the 13-by 9-inch pan for baking lasagna. Add noodles and soak until soft and pliable, 10 minutes; drain.
Meanwhile, mix crème fraiche with half the Parmesan cheese in a small bowl. In a separate large bowl, mix remaining Parmesan with ricotta, minced sage, and the squash, seasoning to taste with salt and pepper.
Spread a little of the crème fraiche mixture over the bottom of the lasagna pan. Arrange in the following order: 3 to 4 lasagna noodles (enough to cover pan) half the squash mixture, half the crème fraiche mixture, remaining squash mixture, another 3 to 4 lasagna noodles, remaining crème fraiche, and finally the sage leaves.
Cover with foil; bake until bubbly, about 25 minutes. Remove foil; broil until spotty brown. Let stand to cool slightly and set; cut into squares and serve.