1 1/2 lb. mild Italian pork sausage, casings
2 Tbs. olive oil
4 garlic cloves, minced
1 1/2 lb. assorted mushrooms, such as
cremini and shiitake, brushed clean and sliced
1/4 cup sherry
8 oz. chopped spinach
9 Tbs. unsalted butter
9 Tbs. all-purpose flour
6 cups milk
Salt and freshly ground pepper, to taste
2 cups ricotta cheese
1 3/4 cups grated Parmigiano-Reggiano cheese
1/4 cup chopped assorted fresh herbs, such as
basil, thyme, rosemary and sage
1/2 tsp. freshly grated nutmeg
2 lb. rigatoni, cooked until tender
2 Tbs. chopped fresh flat-leaf parsley
Preheat an oven to 400°F.
In a large sauté pan over medium-high heat, brown the sausage, stirring and crumbling with a fork, until cooked through, about 10 minutes. Transfer to a bowl.
In the same pan over medium heat, warm the olive oil. Add the garlic and sauté until fragrant and just beginning to brown, 1 to 2 minutes. Add the mushrooms and cook, stirring occasionally, until browned, about 15 minutes. Add the sherry and cook until evaporated, 1 to 2 minutes. Add the spinach and cook until wilted, 30 seconds to 1 minute. Remove from the heat and let cool slightly.
In a small saucepan over medium heat, melt the butter. Add the flour and cook, whisking until light brown and fragrant, about 1 minute. Gradually add the milk, whisking constantly. Reduce the heat to low and whisk until the sauce is smooth and slightly thickened, 4 to 5 minutes. Season with salt and pepper. Remove the béchamel sauce from the heat.
Meanwhile, in a large bowl, stir together the ricotta, egg, 1 cup of the Parmigiano-Reggiano, the assorted fresh herbs, nutmeg, salt and pepper until well blended. Fold in the sausage and the mushroom mixture, then stir in the béchamel sauce and the cooked pasta. Season with salt and pepper.
Spoon the pasta into a 5-quart casserole dish. Sprinkle the remaining 3/4 cup Parmigiano-Reggiano over the pasta. Cover and bake for 45 minutes. Uncover and continue to bake until the top is golden and the cheese is melted, 10 to 15 minutes more. Sprinkle with the parsley. Let stand for 5 minutes before serving.