Grease a 9x13-inch baking dish.

Cook sausage in a large skillet over medium-high heat until evenly browned, crumbly, and no longer pink in the center, 7 to 9 minutes. Drain and discard any excess grease.

While the sausage is cooking, whisk eggs in a mixing bowl until smooth. Whisk in 2 cups milk, mustard powder, and salt until evenly blended.

Spread bread cubes into the prepared dish; sprinkle sausage over top, then Cheddar cheese. Pour egg mixture over the entire dish. Cover and refrigerate, 8 hours to overnight.

When ready to bake, preheat the oven to 300 degrees F (150 degrees C).

Remove casserole from the refrigerator and uncover. Whisk condensed soup and remaining 1/2 cup milk together in a bowl; pour mixture over casserole.