How to Make It
In a medium bowl, toss the 3 cheeses until combined.
Lightly coat the inside of a 6-quart slow cooker with nonstick spray. Add the evaporated milk, whole milk, sour cream, mustard, 1 tablespoon of salt and 1/2 teaspoon each of black pepper and cayenne; whisk to combine. Add the pasta and stir to coat. Reserve 1/2 cup of the grated cheese, then add the remaining cheese to the slow cooker; stir until just combined. Cover and cook on low for 2 hours.
Meanwhile, in a medium skillet, melt the butter over moderate heat. Add the thyme, breadcrumbs, 1 teaspoon of salt and the remaining 1/8 teaspoon of cayenne, if using, and cook, stirring occasionally, until the crumbs are golden brown and fragrant, about 6 minutes. Remove from the heat and stir in the parsley.
Uncover the slow cooker, stir the mac and cheese and top with the reserved 1/2 cup of grated cheese. Cover and cook on low until the cheese is melted and the sauce is thickened, 10 to 15 minutes longer. Top the macaroni and cheese with the seasoned breadcrumbs and serve immediately.