In 10-inch nonstick skillet, cook beef over medium-high heat 5 minutes. Add onion. Reduce heat to medium. Cook 3 minutes longer, stirring occasionally, until beef is thoroughly cooked and onion is tender; drain. Stir in taco seasoning mix, salsa and water. Cook over medium heat 5 minutes, stirring occasionally, until slightly thickened. Spoon hot beef mixture evenly into muffin cups.