Place onions in single layer over bottom of 11-3/4 X 7-1/2-inch microwave-safe dish. Cover with waxed paper; microwave at high 2 minutes.
1 medium onion
Arrange beef strips over onions. Microwave, covered, at medium 5 minutes. Pour off drippings.
Meanwhile, combine ketchup, brown sugar, vinegar, corn starch, dry mustard, hot pepper sauce and salt, mixing thoroughly. Stir into beef mixture; microwave, covered, at medium 8 minutes, stirring after 4 minutes.
1/3 cup ketchup, 2 tbsp brown sugar, 1 tbsp cider vinegar, 1 tsp cornstarch, 1 tsp dry mustard, 1 tsp hot pepper sauce, 1/2 tsp Salt
Arrange muffins on 4 individual plates. Stir beef mixture; spoon an equal portion over each muffin