1. Preheat oven to 375°.
2. Melt butter in a small skillet over medium heat. Add onion and garlic; sauté 5 minutes. Transfer cooked onion to a large bowl; add bread cubes, tossing well to combine.
3. Combine buttermilk and next 5 ingredients in a bowl; pour over bread mixture, stirring well to combine. Stir in Cheddar cheese. Spoon mixture into a 13 x 9-inch baking dish coated with cooking spray.
4. Bake at 375° for 1 hour or until set.