In a large skillet, heat over medium heat and add the medium pork sausage, breaking it apart as it cooks. Add diced onion to the skillet and sauté until the sausage is browned and the onion is translucent, about 5-7 minutes. Drain any excess fat and set the mixture aside to cool.

In a large bowl, whisk together the eggs until well combined, then slowly pour in the whole milk, incorporating it gently. Mix in shredded cheddar cheese, dry mustard, salt, and pepper until fully blended. Carefully fold in the cooled sausage and onion mixture.

Preheat your oven to 350°F before starting the assembly. Grease a 9x13-inch baking dish generously. Spread the bread cubes evenly in the bottom of the dish. Pour the prepared egg mixture over the bread, ensuring that every piece is well-soaked.

Cover the casserole dish tightly with plastic wrap or foil and refrigerate for at least 8-12 hours overnight.

The following morning, preheat your oven to 350°F again if necessary. Remove the cover from the casserole and bake for 50-60 minutes, or until the top is golden brown and the center is set. Let it rest for about 5-10 minutes before slicing.