Butter a 9 x 13-inch pan.

Cover the bottom of the pan with half of the bread cubes.

Layer the broccoli, ham, and cheddar cheese over the bread cubes.

Top with the remaining bread cubes.

In a separate bowl, combine the eggs, milk, salt, nutmeg, dry mustard, and onion flakes (if using).

Pour the egg mixture over the casserole, ensuring all bread is moistened.

Cover the casserole tightly with plastic wrap or foil.

Refrigerate for several hours or preferably overnight.

Preheat oven to 350°F (175°C).

Bake the casserole uncovered for 55 to 60 minutes, or until a knife inserted into the center comes out clean.

Let the casserole stand for 10 minutes before cutting into squares.

Serve warm.