Preheat oven to 450 degrees and butter a 9 by 13-inch glass baking dish.
Bring a large pot of water to a boil and add salt. Cook macaroni according to package directions. Drain.
In a large Dutch oven saute the bacon or ham until done. Remove bacon or ham with a slotted spoon and reserve. Saute the onion until soft. Add the 5 tablespoons butter to the onion mixture and melt the butter stirring with a wooden spoon.
Using a whisk, add the flour, and stir constantly until well mixed with the fat making a roux. Whisk in the mustard. Gradually add the cream whisking constantly.
Add the thyme and salt. Let come to a simmer and stir frequently for 15 minutes.
Working quickly, mix in 1 cup fontina, 1/2 cup Gruyere, 1/2 cup white Cheddar, 1/2 cup Parmesan, the reserved bacon or ham. Continue to stir until all cheese is melted.
Add the cooked noodles to the cheese mixture to coat. Add the noodle mixture to the prepared baking dish.
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