How to Make It
Preheat the oven to 350°. Line a 9-by-13-inch baking pan with foil; lightly grease. In a medium skillet, melt the butter. Add the onion and cook over moderate heat, stirring occasionally, until softened, 8 minutes.
In a small bowl, whisk the tomato sauce with the vinegar, brown sugar, mustard and Worcestershire. In another small bowl, dip the bread in the milk, then gently squeeze out the liquid. Transfer the bread to a large bowl and discard the milk. Add the onion, ground beef, breadcrumbs, eggs, salt, pepper and 1/4 cup of the tomato sauce and mix until just combined; transfer to the pan.
Firmly pat the meat mixture into a 9-by-5-inch loaf; spread the remaining sauce over the top. Bake for 1 hour and 10 minutes, until an instant-read thermometer inserted in the center registers 160°. Let rest for 10 minutes; serve.