Heat oven to 350. Spray a 9" x 13" casserole dish w/ cooking spray. Cook macaroni as directed, drain well.
In lg saucepan over medium heat, heat 3 cups milk, butter, salt and pepper. In small bowl, whisk together 1/4 cup milk and cornstarch, mix well. Whisk cornstarch mix into pan until thickened. Remove from heat, stir in 2 cups cheese until melted.
Pour cheese mix over pasta and stir to combine. Stir in tomatoes. Spoon into casserole dish. Top w/ remaining cheese. Bake 40 mins.