In a large skillet, set over medium heat, add the breakfast sausage and break it into small pieces with a spatula. Cook until the sausage is browned and no longer pink, approximately 6-8 minutes. Stir in the dried minced onion during the last minute of cooking for flavor. Once done, drain any excess fat and set the sausage mixture aside.

In a large mixing bowl, crack all 12 large eggs and whisk them until smooth and well combined. Gradually mix in the whole milk, salt, and ground black pepper, ensuring everything is incorporated.

Preheat your oven to 350°F (175°C) and grease a 9x13 inch casserole dish with cooking spray or butter. Spread about 3/4 of the cooked sausage mixture evenly at the bottom of the dish. Next, sprinkle half of the shredded sharp cheddar cheese on top of the sausage layer, which adds a cheesy depth before pouring in your egg mixture.

Carefully place the casserole in the preheated oven and bake for 30 minutes or until the center is mostly set but slightly jiggles. Remove it from the oven, sprinkle the remaining sausage and cheese evenly on top, then return it to the oven for an additional 10-15 minutes, until fully set.

Let the casserole cool for about 10 minutes to make slicing easier. Cut it into squares and serve it warm, either on its own or with sides like fresh fruit salad.