2) In a large Dutch oven or other heavy pan, saute the bacon until crisp. Remove the bacon with a slotted spoon and reserve. Saute the onion in the bacon drippings until soft. Add the butter to the onion mixture and melt, stirring with a wooden spoon.
3) 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 milk and cream, whisking constantly.
4) Add the thyme, bay leaf and salt. Let the mixture come to a simmer and stir frequently for 15 minutes. Strain the hot milk mixture into a metal bowl and discard the solids.
5) Working quickly, mix in the blue cheese, and 3/4 each of the fontina, gruyere, white cheddar and Parmesan. Stir in the reserved bacon and the parsley. Continue to stir until all the cheese is melted.
6) Add the cooked pasta to the cheese mixture and mix to coat. Pour the pasta mixture into the prepared baking dish. Mix the remaining cheese and breadcrumbs together and sprinkle on top of the pasta.
7) Bake for about 15 to 20 minutes or until bubbling and golden brown. Remove from the oven when done and rest for 5 minutes before serving