Saute the onions, garlic and peppers in a large cast iron skillet with 2 TBSP vegetable oil for 5-7 minutes. Remove, set aside and allow to cool. Butter each slice of bread on both sides. Place the bread slices in the bottom of a 9×13 casserole dish.
Whisk together your eggs,milk, cheese and seasonings. Toss in the sauteed onions, peppers and garlic. Pour over the bread slices and top with the crumbled sausage. Refrigerate over night or at least 6 hours. I prepared this in the morning and left it in the fridge until dinner-time rolled around. Bake at 350 until eggs are set, about 45 to 50 minutes.