1Grease an 8 1/2x11-inch baking dish with butter. Place the bread cubes in the baking dish.
2Break the sausage into small pieces into a large skillet over medium heat. Cook and stir sausage until completely browned, 5 to 7 minutes. Add green onions; toss once and remove skillet from heat. Drain as much grease as possible from the skillet. Spread sausage and green onion mixture over bread cubes.
3Stir cream of mushroom soup and 1/2 cup milk together in a bowl; pour over the layer of sausage in the baking dish.
4Whisk eggs and milk together in a bowl; pour over the sausage. Top with Gruyere and Cheddar cheese. Cover the dish with aluminum foil and refrigerate 8 hours to overnight.
5Preheat oven to 350 degrees F (175 degrees C).
6Bake uncovered in preheated oven until the middle is set, about 1 hour.