Preheat the oven to 350 F. Line a large baking pan or a rimmed baking sheet with parchment paper if you have it. Either way, spray the baking sheet with nonstick spray.
In a large, heavy skillet, melt the butter over medium heat. Add the onions, scallions, carrots, and garlic. Sauté until the vegetables are golden and tender, about 6 minutes, stirring often. Cool completely to room temperature.
In a large bowl, combine the sauteed vegetables, salt, pepper, cayenne, eggs, and mustard or horseradish. Thoroughly blend in ½ cup of the ketchup and milk or half and half. Add the beef, pork, and breadcrumbs to the bowl, and use your hands to gently but thoroughly combine the meatloaf mixture. I like to use my hands for all meatloaf mixings, which allows the ingredients to become blended without crushing the meat. Form the mixture into a loaf shape.
Place the meatloaf on the prepared baking dish. Brush the remaining 1/2 cup of ketchup evenly over the top and side of the meatloaf in a thin glaze. Place the three strips of bacon diagonally across the meatloaf, if using, and bake for 50 to 60 minutes until the meatloaf is cooked through (160-degree internal temperature on a meat thermometer) and the top and sides have browned up a bit.
Let the meatloaf rest on the baking sheet for 20 minutes before slicing and serving.
Notes
And the next day — meatloaf sandwiches! I had an old friend who was a sophisticated cook but relished a good meatloaf sandwich above almost everything else. I think of him every time I make one. Yes, it’s the ultimate comfort food for some. But comfort does not mean boring.