Preheat oven to 400 F degrees. Butter or grease a 9x13 inch casserole dish.
Cook pasta according to package instructions.
Add the pancetta to a saucepan and cook until it browns. Remove the pancetta from pan and place over paper towels to soak up some of the fat.
Separate the eggs and whisk the egg whites until stiff peaks form. In another bowl whisk the egg yolks and set aside.
Add the butter to the saucepan and melt. Add the minced garlic and cook until it starts foaming and gets aromatic, about a minute. Sprinkle the flour over the garlic and stir. Whisk in the milk and cook until the sauce thickens and coats the back of a wooden spoon.
Season the sauce with salt, pepper and nutmeg.
Add half the gruyere and half the cheddar cheese to the sauce and continue whisking until the cheese melts.
Temper the egg yolks by adding a ladle of the sauce at a time to the egg yolks and whisking them, add about 3 ladles of sauce altogether. You want to do this to avoid the eggs from scrambling up when you add them to the cheese sauce.
Stir in the egg yolk mixture back into the cheese sauce. Remove from heat. Fold the egg whites into the cheese sauce.
Start layering the casserole by adding half the pasta shells to the prepared baking dish. Sprinkle half the pancetta over the pasta, then add half the cheese sauce. Sprinkle half of the remaining gruyere and white cheddar over then add the rest of the pasta. Repeat with the remaining pancetta, sauce, and cheese. Finally top with the chopped parsley and place the dish in the oven.
Bake for 20 minutes until golden and puffed. If you want it more golden you can turn the broiler on and broil the souffle for 2 or 3 minutes.