Begin by preheating your oven to 350°F. Position a rack in the middle of the oven to ensure even baking of the meatloaf later on.

Melt the unsalted butter in a large, heavy-bottomed skillet over medium heat. When the butter starts to foam, add the chopped yellow onion, celery, carrot, green onions, and minced garlic. Cook for about 5 minutes, stirring occasionally. Then, cover the pan and continue cooking for another 5 minutes until the carrots are tender, stirring now and then. Season the vegetables with salt and ground black pepper. Add Worcestershire sauce and 1/3 cup ketchup, then cook for 1 more minute. Remove the pan from heat and let the mixture cool. I always let the vegetables cool completely so they don't start cooking the eggs in the next step.

Once the vegetable mixture from Step 2 has cooled to the touch, transfer it to a large mixing bowl. Add the ground beef, hot pork sausage (or Italian sausage), lightly beaten eggs, fresh bread crumbs, and chopped parsley. Using clean hands, gently but thoroughly mix everything together until evenly combined, but take care not to overwork the mixture.

Transfer the meatloaf mixture from Step 3 into a loaf pan (4x8-inch or 5x9-inch) and press it down to compact it evenly. Alternatively, form a free-standing loaf on a rimmed baking pan. Spread the remaining 1/3 cup ketchup evenly over the top of the loaf. Personally, I like to add a little extra ketchup on top for a more caramelized finish.

Bake the prepared meatloaf at 350°F for 1 hour, or until a meat thermometer inserted in the center reads 155°F. Remove the meatloaf from the oven and let it rest for 10 minutes before slicing. Resting allows the juices to redistribute, making the meatloaf moist and easier to cut. I always let it rest—it's the secret to juicy slices!

Use a metal spatula to gently lift the meatloaf out of the pan and transfer to a serving plate. Slice into thick pieces and serve—enjoy your homemade meatloaf! Leftovers keep well; they're delicious reheated or even cold in a sandwich.