Bring water to a boil in a Dutch oven. Add macaroni, carrot, and broccoli; cook 8 minutes or until macaroni is almost tender. Drain.
Return macaroni mixture to Dutch oven; add 1 1/4 cups cheese and next 3 ingredients; toss lightly.
Combine flour and 3 tablespoons milk in a small bowl; stir well with a wire whisk. Stir in remaining milk. Stir milk mixture into macaroni mixture. Spoon mixture into an 11- x 7- x 1 1/2-inch baking dish coated with cooking spray. Cover and bake at 350° for 25 minutes. Uncover and sprinkle remaining 1/2 cup cheese over top of casserole. Bake 5 additional minutes or until cheese melts.