Preparation

Make the Topping:

• 如果使用大蒜，请将其搅拌成黄油。将面包屑放在一个中等大小的碗中，将黄油淋上碎屑。加入少许盐和帕马诺 - 雷吉亚诺（如果使用）。用双手折腾，结合在一起。搁置。

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, Fontina, Parmigiano-Reggiano, sage, and thyme. Season to taste with salt. Keep warm.
• Bring a large pot of well-salted water to a boil over high heat. Add the pasta shells 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, Brussels sprouts, and pancetta 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.