This is a very simple way to liven up mac 'n' cheese. Throw in some bacon and frozen peas, and boom: instant elevation. The bacon and peas have that lovely salty-sweet combination, with peas bringing a little bit of freshness to this decadent dish.
Preheat the oven to 375 degrees F. Butter a 2-quart baking dish or eight 12-ounce ramekins.
For the bechamel: Heat a medium saucepan over medium heat. Add the bacon and cook, stirring often, until just crisp, about 5 minutes.
Meanwhile, combine the 3 1/2 cups milk, the parsley, peppercorns, crushed red pepper, bay leaf and nutmeg in another medium saucepan. Bring to a simmer over medium heat, then remove from the heat and keep warm.
When the bacon is crisp, remove it with a slotted spoon and drain on a paper towel-lined plate, leaving 6 tablespoons of the bacon fat in the saucepan. If you have less than 6 tablespoons, make up the difference with the butter and heat over medium heat until melted. Add the flour and cook, stirring, until the mixture is doughy and smells a bit nutty, about 1 minute.
Strain in the warm milk about a half cup at a time into the flour mixture and cook, whisking to incorporate after each addition. The bechamel should be smooth and velvety. If it's chunky or too thick, whisk in some more milk.
For the mac 'n' cheese: Remove the bechamel from the heat and add all of the cheeses; whisk to combine. Fold in half of the bacon. Next, fold in the macaroni, then the peas. Transfer to the prepared baking dish or ramekins. If using ramekins, place them on a rimmed baking sheet.
Combine the breadcrumbs with the remaining bacon pieces, the melted butter and a pinch each of salt and black pepper in a small bowl. Sprinkle over the mac 'n' cheese and bake until the sauce is bubbly and the crust is browned on top, 30 to 45 minutes (depending on the baking vessel).