Preheat your oven to 350°F. Grease a 9×13-inch baking dish with butter. This prevents sticking and helps the casserole develop a nice golden crust while baking.

Place a frying pan over medium heat. Add the breakfast sausage. Cook until brown and fully cooked. Break it into small crumbles while cooking so it spreads evenly in the casserole.

Place the bread cubes into the greased baking dish. Spread them evenly across the bottom. The bread will absorb the egg mixture and create a soft, fluffy texture.

Sprinkle the cooked sausage over the bread cubes. Add chopped onion evenly across the dish. This layer adds flavor and makes every bite more delicious.

Sprinkle cheddar and mozzarella cheese over the sausage mixture. Spread the cheese evenly so it melts smoothly during baking and creates a creamy texture.

In a large mixing bowl, crack the eggs. Add milk, salt, pepper and garlic powder. Whisk well until everything blends together smoothly.

Slowly pour the egg mixture over the bread, sausage and cheese layers. Make sure all the bread cubes absorb the liquid mixture evenly.

Allow the casserole to rest for about 10 minutes before baking. This helps the bread soak up the egg mixture and improves the texture.

Place the baking dish in the oven. Bake for about 40 minutes. The top should turn golden and the center should set completely.

Remove the casserole from the oven. Let it cool for about 5 minutes. Slice into squares and serve warm for the best flavor.