Heat the butter in a large skillet over medium-high heat. When the foaming subsides, add the beef, onions, salt, and pepper.
Cook, breaking up the beef into small pieces, until the beef is no longer raw and onions are soft, about 5 minutes. There's no need to drain the mixture.
Reduce the heat to medium and add the garlic. Cook, stirring, 1 more minute.
Stir in the ketchup, balsamic vinegar, and sweetener. Lower the heat to medium-low and simmer, uncovered, stirring often, for 5 more minutes. Serve immediately.
While you definitely want ground beef to be fully cooked, you don't want it overcooked, because then it turns dry and mealy instead of juicy.So in the first step of cooking the beef with the onions, you should only cook it until it's no longer raw - it's OK if it's still slightly pink. It will cook further during the next stages.