Ham and Egg Casserole

2 tablespoons butter or margarine
1 onion, thinly sliced
2 tablespoons all-purpose flour
1 cup milk
1 tablespoon mustard
1/4 teaspoon ground black pepper
2 cups cubed cooked ham
2 hard-boiled eggs, sliced *
*If you don't have hard-boiled eggs on hand, start cooking them 5 minutes before you turn on the oven and allow 15 minutes for cooking.
Heat oven to 350 degrees F.
Melt butter in a 1-quart saucepan over medium heat. Add the onion and cook, uncovered, until soft, about 5 minutes. Reduce the heat to low. Blend in the flour and slowly add the milk. Cook, stirring constantly, until the sauce is thickened and smooth, about 3 minutes.
Remove the pan from the heat and stir in the mustard and pepper.
Pour half of the sauce into an ungreased 1-quart casserole; add a layer of half of the ham, then a layer of half of the sliced eggs.
Repeat the layers of ham and eggs, then pour the remaining sauce over top.
Bake, uncovered, about 20 minutes or until the sauce is bubbly.