In 12-inch nonstick skillet, cook beef over medium-high heat, stirring frequently, until thoroughly cooked. Drain.
Reserve 2 tablespoons onions for garnish; stir remaining onions, the soup and salsa into beef. Heat to boiling. Reduce heat to medium-low; cook 5 minutes.
Stir in cooked pasta; cook 3 to 5 minutes, stirring occasionally, until thoroughly heated. Sprinkle with cheese; cover and cook just until melted. Sprinkle with reserved 2 tablespoons onions.