1. Coat inside of slow cooker with cooking spray. Cook bacon in a large skillet over medium-high heat until browned and crisp, 5 to 7 minutes. Transfer bacon to a paper towel-lined plate and pour off all but 1 Tbsp. fat from skillet. Add onion, bell pepper and celery to skillet. Cook, stirring often, until softened, 3 to 5 minutes. Add garlic and sauté 1 minute longer. Remove from heat. Coarsely chop bacon.
2. In a large bowl, whisk eggs, milk, salt and pepper. Add bread cubes and stir in onion mixture, bacon, cheese and tomatoes. Stir well and pour into slow cooker. Cover and cook on low until a knife inserted near center comes out clean, 3 to 3 1/2 hours.