Meanwhile, finely slice white parts of onions; set aside. Finely slice green parts of the onions; set aside. In a large skillet, cook beef over medium-high heat, breaking up with spoon, until no longer pink, about 5 minutes. Drain off any fat. Stir in white parts of onions, plus garlic, chili powder and vinegar, and salt and pepper to taste; cook for 1 minute. Stir in tomato juice; cook, stirring often, until liquid is reduced by half and sauce is thickened, about 3 minutes.