1-First Step: Prep Your Oven and Baking DishPreheat your oven to 350°F (175°C). Grease a 9×13-inch (23×33 cm) baking dish well with cooking spray or olive oil to prevent the casserole from sticking during baking.

2-Second Step: Cook the SausageHeat 1 tablespoon olive oil in a large skillet over medium-high heat until shimmering. If your breakfast sausage has casings, remove them. Add the sausage to the pan and cook, breaking it into small pieces, for about 4 to 7 minutes until fully browned and cooked through. Drain any excess grease and set the sausage aside to cool slightly.

3-Third Step: Sauté Optional VegetablesIf you decide to include vegetables, such as diced green and red bell peppers and chopped green onions, add them to the skillet next. Sauté for 2 to 3 minutes until tender and aromatic. Then, remove from heat and set aside.

4-Fourth Step: Prepare the Egg MixtureIn a large bowl, vigorously whisk together 12 large eggs, 2 cups half-and-half (or the sour cream and milk alternative), 2 teaspoons dry mustard (if using), 1 teaspoon kosher salt, and 1/2 teaspoon freshly ground black pepper. Whisk until thoroughly combined and slightly frothy to help create a fluffy custard.

5-Fifth Step: Combine Bread, Cheese, and EggsAdd half (about 4 ounces) of the shredded sharp cheddar cheese and the 6 cups of bread pieces into the egg mixture. Gently fold the ingredients until the bread is coated and beginning to absorb the custard.

6-Sixth Step: Add Sausage and VegetablesUsing a slotted spoon, fold the cooked sausage and sautéed vegetables (if used) into the egg-bread-cheese mixture. Mix gently but evenly to distribute all ingredients.

7-Seventh Step: Assemble and RefrigerateTransfer the mixture into the prepared baking dish and spread evenly for consistent cooking. Sprinkle the remaining shredded cheddar cheese evenly over the top. For best texture and flavor, cover and refrigerate the casserole for at least 8 hours or overnight. This soaking period allows the bread to absorb the custard fully, and the flavors to meld beautifully.

8-Final Step: Bake and ServeBefore baking, let the refrigerated casserole come to room temperature while the oven preheats. Bake uncovered on the middle rack at 350°F (175°C) for 50 to 55 minutes or until the top turns golden brown and a knife inserted into the center comes out clean. Expect the edges to be set firmly; the center might still jiggle slightly but will firm up as it cools. Allow the casserole to cool for about 10 minutes to set before slicing and serving. Garnish with extra chopped green onions or herbs if desired.