Preparation

Preheat oven to 350 degrees. On a large baking sheet, toss together the butternut squash, 3 tablespoons of olive oil, 1 1/2 teaspoons of kosher salt and 1/2 teaspoon of freshly cracked pepper. Bake for 25-30 minutes, or until soft.
While the squash cooks, heat the other tablespoon of olive oil and butter in a large sauté pan over medium heat. Add the onions, a heavy dash of kosher salt and pepper, and the maple syrup. Cook until brown and caramelized. Add the sage and cook for one more minute.
In a large mixing bowl, add all but 1/2 cup of the cooked butternut squash and roughly mash with a fork. Then add the caramelized onion, ricotta, and parmesan. Stir together until combined. In a medium mixing bowl, whisk together the creme fraiche, anchovy paste, and nutmeg with 1/2 cup of cold water. Continue adding water until the mixture is smooth and has the consistency of white pasta sauce - it should be thin but not watery.
Turn your oven temperature up to 400 degrees, then build the lasagna. Start by greasing an 8x12-inch baking dish, then spoon a thin layer of white sauce evenly into the bottom. Top with the lasagna noodles, a layer of butternut squash mixture, and a thin layer of white sauce. Repeat this process 3 more times, ending with a white sauce on top. Evenly sprinkle the fontina, mozzarella, and reserved squash cubes over the top. Top with pieces of the prosciutto. Bake for 30 to 35 minutes or until golden and bubbly.