Heat a large skillet over medium heat. Add the ground beef and chopped onion, then cook for about 6 to 8 minutes, breaking up the beef as it browns, until the meat is no longer pink and the onion is tender.

Drain off excess fat from the skillet before adding the sauce ingredients. This keeps the finished dish from becoming greasy and helps the sweet-tangy flavors stay concentrated.

Stir in the pork and beans, tomato sauce, ketchup, packed brown sugar, prepared yellow mustard, salt, and black pepper. Mix gently until the beef, beans, and sauce are evenly combined.

Bring the mixture to a gentle boil over medium heat. Watch the edges of the skillet so the sauce begins bubbling without scorching.

Reduce the heat and simmer uncovered for about 10 minutes, stirring occasionally. Continue until the skillet is hot and the sauce has reduced slightly for a thicker, more cohesive consistency.

Serve immediately while warm. Spoon the beef and bean mixture alongside cornbread, toast, biscuits, baked potatoes, or a simple green salad.