This can be mixed up the night before and left in the refrigerator until morning, which makes it perfect for a New Year's Morning brunch. Adapted from a recipe by Sydney Oland at Serious Eats http://bit.ly/g4Wuk7
Preheat oven to 375°F degrees. Cut loaf of bread into 1 inch slices, then roughly tear into large pieces (around 3 inches). Place on baking sheet and toast in oven for 10 minutes tossing once. (You may skip this step if using day old bread, as long as it's fairly dry.).
Heat large skillet over medium high heat, crumble loose sausage into pan and cook until no longer pink (about 7-9 minutes). Remove sausage from skillet and place in bowl large enough to accommodate sausage and bread.
Pour off and discard all but 1 tablespoon of fat from skillet and add onion. Cook over medium low heat until soft and brown, 5-7 minutes. Add onion and torn bread to sausage and toss. Add 1 cup of shredded cheddar cheese and toss again.
Butter a 9 x 13 x 2 inch Pyrex baking dish. Beat eggs well then beat in milk and sour cream til smooth; season with salt and pepper.
Place sausage/bread/onion/cheese mixture into buttered dish and pour milk/egg mixture over, pressing down on the bread with a spatula to insure that the egg begins to soak up the liquid. Sprinkle with remaining shredded cheese and scallion (if using), cover and place in fridge overnight.
The following morning (or early afternoon) preheat oven to 350°F Bake until browned and puffy, about 45-50 minutes. Serve hot with a Bloody Mary. Pass sour cream, hot sauce in general (sriracha specifically), ketchup, salsa, or sliced avocados; if you have the energy, top each serving with a runny fried egg.