- 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.