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1
Preheat the oven to 375 degrees. Grease a 13x9-inch baking pan. Bring a pot of salted water to a boil over high heat. Cook the pasta until it is just beginning to soften to the al dente stage. Drain the pasta, run it under cold water, and return it to the pot.
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2
Scrape the white rind off of the Brie, and chop it finely. Combine the Brie with the Gruyere and cheddar, and set aside 1/2 cup of the cheese mixture.
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3
Heat 2 tablespoons of the butter in a saucepan over medium-low heat. Stir in the flour and cook, stirring constantly, for 1 minute, or until the mixture turns slightly beige, is bubbly, and appears to have grown in volume. Stir in the rosemary, sage, thyme, and nutmeg. Increase the heat to medium, and slowly whisk in the warm milk. Bring to a boil, whisking frequently. Reduce the heat to low, and simmer the sauce for 2 minutes.
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4
Add the cheese to the sauce by 1/2-cup measures, stirring until the cheese melts before making another addition. Pour the sauce over the pasta, and stir well. Season to taste with salt and pepper, and transfer the pasta to the prepared pan.
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5
Heat the remaining butter in a small skillet over medium heat. Add the breadcrumbs, and cook for 2 minutes, or until the crumbs are golden brown. Combine the browned crumbs with the remaining cheese mixture, and sprinkle it over the top of the dish.
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6
Bake the casserole for 20 minutes to 30 minutes, or until the cheese sauce is bubbly and the crumbs on the top are deep brown. Allow to sit for 5 minutes, then serve.
