COOK macaroni in large saucepan as directed on package, omitting salt. Remove from pan; drain.
ADD chili to same saucepan; cook on medium-high heat 5 to 8 min. or until heated through, stirring frequently. Stir in macaroni and 1/4 cup VELVEETA; cook 5 min. or until VELVEETA is completely melted, stirring occasionally.
SERVE macaroni topped with remaining VELVEETA, tomatoes and onions.