01 - Preheat oven to 375°F (190°C). Line a 9x5-inch loaf pan with parchment paper or lightly grease.
02 - Boil elbow macaroni according to package directions until al dente. Drain well and set aside.
03 - In a saucepan over medium heat, melt butter. Whisk in flour and cook for 1 minute. Gradually whisk in whole milk and cook until the mixture thickens, about 3 to 4 minutes.
04 - Remove sauce from heat. Stir in shredded cheddar, ground mustard, cayenne if desired, and season with salt and pepper. Fold in cooked macaroni. Allow to cool slightly.
05 - In a large bowl, thoroughly combine ground beef, ground pork, breadcrumbs, eggs, onion, garlic, milk, ketchup, Worcestershire sauce, dried thyme, salt, and pepper until just incorporated.
06 - Press half of the meat mixture evenly into the bottom of the prepared loaf pan, leaving a shallow well in the center.
07 - Spoon mac and cheese mixture into the well, leaving a ½-inch border around all sides to prevent leakage.
08 - Spread remaining meat mixture over the mac and cheese filling, sealing edges well to fully enclose the center.
09 - Transfer loaf pan to oven and bake for 50 minutes.
10 - Remove loaf from oven, sprinkle with remaining shredded cheddar, and return to oven for an additional 10 minutes until cheese is melted and meatloaf reaches 160°F (71°C) internally.
11 - Let loaf rest for 10 minutes before slicing. Garnish slices with chopped parsley if desired.