Preheat the oven to 350F. Grease a 9x13 baking dish.
In a heavy skillet, cook the sausage with a little vegetable oil, breaking up the sausage with a spoon, until cooked and browned. Remove from the pan.
1 pound pork breakfast sausage
In the same skillet, cook the onion until translucent, about 4-5 minutes. Remove from the pan and set aside.
1 medium onion
In a large mixing bowl, whisk together the eggs, ½ the cheese, milk, and seasoning until combined. Stir in the toasted bread, cooked sausage and onions. Pour into the prepared baking dish, top with the remaining cheese, cover, and refrigerate at least 8 hours or preferably overnight.
8 large eggs, 2 cups shredded cheddar cheese, 2 cups whole milk, 1 teaspoon kosher salt, ½ teaspoon black pepper, 6 slices white bread
Let the casserole come to room temperature before baking for 40-50 minutes, uncovered, or until a cake tester comes out clean.