Heat a large skillet over medium heat. Add the breakfast sausage and chopped onions, cooking together for 6–8 minutes until browned and onions are tender. Drain excess fat.

In a mixing bowl, whisk together eggs, milk, mustard powder, salt, and black pepper until well combined.

Grease a 9x13-inch baking dish. Place half of the cubed bread at the bottom, add the sausage mixture and half the shredded cheese. Layer remaining bread, sausage, and top with remaining cheese.

Pour the egg mixture over the layers in the baking dish. Press down to ensure bread absorbs the custard. Cover and refrigerate overnight.

Preheat oven to 350°F (175°C). Bake covered for 45–50 minutes until top is golden and a knife inserted comes out clean. Allow to cool slightly before serving.