Preparation

Make the Topping:

• If using the garlic, stir it into the butter. Put the breadcrumbs in a medium bowl and drizzle the butter over the crumbs. Add a pinch of salt and the Parmigiano-Reggiano or pecorino (if using). With your hands, toss well to combine. Set aside.

Make the Macaroni:

• Position a rack in the center of the oven and heat the oven to 350°F.
• Melt the butter in a heavy-duty 3- to 4-quart saucepan. Whisk in the flour and continue to whisk over low heat for 3 minutes—the butter and flour should gently bubble and froth without coloring. Slowly add the milk, while whisking constantly. Be sure to whisk around the edges of the pan to get all the lumps. Whisk until the sauce is smooth and has the consistency of heavy cream, about 6 minutes. Raise the heat to medium and bring the sauce to a simmer, whisking constantly. Lower the heat to maintain a gentle simmer and cook, whisking occasionally, for 10 minutes.
• Turn off the heat and gently whisk in the Gruyère, mascarpone, and rosemary. Season to taste with salt. Keep warm.
• Bring a large pot of well-salted water to a boil over high heat. Add the penne and cook, stirring occasionally, until al dente, about 1 minute less than indicated on the package. Drain well and return to the pot.
• Add the cheese sauce, mushrooms, and prosciutto to the pasta and gently stir to combine. Transfer the pasta mixture to a 9×13-inch baking dish and evenly scatter the reserved breadcrumbs on top. Bake at 350°F until the top is golden, about 15 minutes. Serve immediately.

For individual servings: choose small baking dishes that hold 1-1/2 to 2 cups each. You can fill them to within about 1/2 inch of the rim. The number of servings will, of course, depend on the volume of your baking dishes.