1 tablespoon olive oil
1 yellow onion, diced
4 garlic cloves, minced
Salt and freshly ground pepper, to taste
1 pound mild Italian sausage, casings removed
4 tablespoons (1/2 stick) unsalted butter
1/4 cup all-purpose flour
3 cups milk
1 cup grated Parmigiano-Reggiano cheese
1/4 cup plus 1 tablespoon chopped fresh flat-leaf parsley
1 tablespoon minced fresh sage
9 dried lasagna noodles, cooked al dente
3 pound butternut squash, neck portion only, peeled and sliced into wide, thin sheets
Position a rack in the lower third of an oven and preheat to 400°F.
2In a large nonstick fry pan over medium-high heat, warm the olive oil. Add the onion and cook, stirring occasionally, until soft and translucent, about 8 minutes. Add the garlic, salt and pepper and cook for 30 seconds. Transfer to a bowl.
3In the fry pan over medium-high heat, cook the sausage, breaking it into small pieces, until browned and cooked through, about 8 minutes. Drain the sausage on paper towels, then add to the onion mixture.
4In the fry pan over medium heat, melt the butter. Add the flour and cook, stirring constantly, for 1 minute. Slowly stir in the milk, increase the heat to medium-high and cook until thickened, 8 to 10 minutes. Transfer to a separate bowl.
5Stir in 3/4 cup of the cheese, the 1/4 cup parsley, the sage, salt and pepper.
6To assemble the lasagna, spread 1/3 cup sauce on the bottom of the fry pan and top with a single layer of 3 noodles. Top with 1/3 cup sauce, 1/3 cup sausage and a layer of squash slices. Repeat the layering of sauce, sausage and squash 5 more times, replacing the squash layer with a layer of noodles after every 2 layers of squash. Top with the remaining sauce and the 1/4 cup cheese.
7Transfer the pan to the oven and bake until the lasagna is browned and bubbly, about 45 minutes.
8Sprinkle with the 1 tablespoon parsley. Let the lasagna stand for 15 minutes before slicing.