HEAT oven to 350°F.
BRING broth and water to boil in medium saucepan. Add macaroni; cook 6 min. or just until tender. Meanwhile, melt butter in large saucepan on low heat. Stir in flour and mustard; cook and stir 2 min. or until bubbly. Gradually stir in milk. Blend in sour cream. Bring to boil on medium heat, stirring constantly; simmer on medium-low heat 3 to 5 min. or until thickened, stirring constantly. Gradually stir in 1-1/2 cups cheese; cook and stir 3 to 5 min. or until melted.
DRAIN macaroni. Add to cheese sauce; mix lightly. Spoon into 1-1/2-qt. casserole sprayed with cooking spray; top with remaining cheese.
BAKE 20 min. or until heated through.