Cook and drain noodles according to package directions.
Combine milk, corn starch, salt, mustard and pepper in a 3-quart saucepan. Add margarine. Bring to a boil over medium heat, stirring constantly; boil 1 minute. Remove from heat. Stir in 1-3/4 cup cheese into sauce until smooth; stir in noodles and salsa. Turn into 2-quart casserole; sprinkle with reserved cheese.