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1
Heat oven and boil pasta Heat oven to 375F.
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2
Bring a large pot of water to a boil, then add salt generously and cook pasta 2 to 3 minutes less than manufacturers instructions (the outside should be cooked but the inside underdone).
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3
Transfer to a colander, rinse under cold running water, and drain well.
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4
Meanwhile, make bread crumbs Tear bread into large pieces and pulse a few times in a food processor to form very large crumbs.
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5
Transfer to a bowl, and add melted butter.
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Toss to evenly coat.
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7
Prepare baking dishes Butter eight 6-ounce shallow baking dishes or a 1 1/2-quart baking dish.
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8
Make cheese sauce Melt butter in a 4-quart pot over medium heat, then sweat onion until translucent, stirring occasionally, about 5 minutes.
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9
Whisk in flour and cook, stirring with a wooden spoon, until bubbling but not browning, about 45 seconds.
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10
Add milk and whisk to combine.
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Bring to a simmer, stirring with a wooden spoon (scrape across the bottom and around edge of pot to prevent scorching), until thickened, about 4 minutes.
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12
Add fontina, 2/3 cup grated Gruyere, 1 2/3 cups grated Cheddar, and 1/2 cup Parmigiano-Reggiano, stirring until completely melted and sauce is smooth.
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13
Season with salt and pepper, add cayenne and nutmeg, and stir to combine.
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14
Assemble and add cheese topping Add pasta to sauce and stir to thoroughly combine.
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15
Pour into prepared baking dishes and sprinkle evenly with the reserved cheeses, followed by the bread crumbs.
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16
If using, top with roasted tomato slices and thyme.
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Bake Place dishes on a parchment-lined baking sheet and bake until bubbling and cheese is golden brown, 25 to 30 minutes.
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18
Let cool 5 minutes before serving.
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19
Cut 6 small tomatoes into 1/4-inch-thick rounds (about 24 total) and arrange in a single layer on rimmed baking sheets.
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20
Drizzle with 2 tablespoons extra-virgin olive oil and season with coarse salt and freshly ground pepper.
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21
Sprinkle with 1 teaspoon fresh thyme leaves.
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22
Roast at 400F until softened and browned in spots, about 20 minutes.
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23
Let cool before storing.
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24
Using a variety of cheeses offers the best balance of flavors.
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25
If you want to substitute those suggested below with others, consider their flavors and melting qualities.
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26
Combine pungent cheeses, such as sharp Cheddar and Gruyere, with milder ones like fontina or Monterey Jack, then add Parmigiano-Reggiano or pecorino Romano for extra bite.
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27
A good Italian brand of dried elbow macaroni will have the best consistency.
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28
Undercook your pasta so that it is the slightest bit crunchy in the center, then rinse it under cold water.
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29
This stops the cooking and washes off the extra starch, which, contrary to what you might think, is not useful in thickening the casserole; instead, as it bakes, that extra starch merely expands and lends a mealy texture to the sauce.
