Place bread cubes on rimmed baking sheet; bake 10 min. or until lightly toasted. Meanwhile, cook and stir bacon and onions in skillet on medium heat until bacon is browned and onions are tender.
Combine bread cubes, onions and bacon in large bowl. Stir in tomatoes. Spoon half the bread mixture into 13x9-inch baking dish sprayed with cooking spray; top with half the cheese slices. Repeat layers. Whisk eggs and milk until well blended; pour over bread mixture.
Bake 35 min. or until center is set. Let stand 5 min. before serving.