Prepare pasta according to package directions. Drain. Set aside.
Place a large heave skillet over medium heat.
When hot add bacon. Cook until crispy.
Using a slotted spoon or tongs, remove cooked bacon from the skillet to a paper towel-covered plate to drain. Set aside.
Reserve fat in skillet. Do not pour out.
Once bacon has cooled, chop into small pieces.
Add butter to the leftover bacon grease. Melt.
Slowly add in the flour, whisking until smooth before adding more.
Over medium heat, cook until mixture turns a light, golden color and is the consistency of wet sand, about 1 - 2 minutes.
Slowly add in milk, whisking continuously until very smooth.
Bring to a boil over medium-high heat.
Add the bay leaf. Cook 5 minutes, whisking frequently.
Add in all cheeses, one at a time. Whisk until each is fully incorporated into the sauce.
Add yellow mustard, nutmeg, cooked pasta and bacon. Stir well to combine.
Taste. Add salt and pepper, to taste.
Pour into a serving dish.
Serve hot.