Preheat oven to 350 F.

Prepare the mushrooms: Place 1 tablespoon butter in a large skillet and melt over medium heat. Add half the chopped onion. Cook over medium heat 2 – 3 minutes until soft, but not browned. Add half of the mushrooms and cook for 2 minutes. Add 1 tablespoon sherry, 1 tablespoon thyme, and salt and pepper to taste. Cook until the mushrooms are just tender. Remove from heat and transfer to a large bowl. Repeat with the rest of the mushrooms.

Prepare the cheese sauce: In a medium saucepan, melt 2 tablespoons butter over medium heat. Whisk in the flour and cook about 1 minute. Gradually add milk and cream, whisking continuously. Cook over medium heat until the milk begins to thicken, whisking all the while. Once thickened, remove from heat and add the Gruyere, stirring until smooth and melted. Add nutmeg and season with salt and pepper.

In a large bowl, mix together the cooked pasta, mushroom mixture, spinach, prosciutto and cheese sauce. Transfer to casserole dish. Cover and bake 20 minutes.

In a small bowl, mix together breadcrumbs, remaining tablespoon of thyme and Parmesan. Melt the remaining 2 tablespoons of butter and toss with the breadcrumb mixture.

Remove pasta from the oven and sprinkle the breadcrumb mixture over the top. Increase oven temperature to 375 F and bake 15 – 20 more minutes, uncovered, until golden and bubbly.