Cheesy Ham and Potato Casserole

Preheat the oven to 350°F and lightly grease a 2-quart baking dish; set it aside.
Melt the butter in a large skillet or saucepan over medium heat. Add the chopped onion and cook, stirring occasionally, until the onion is tender but not browned, about 5 minutes.
¼ cup butter, ½ medium onion
Stir in the flour, salt, dry mustard, and pepper until the onions are well coated. Let the mixture cook for 1 to 2 minutes to remove the raw taste from the flour.
¼ cup all-purpose flour, 1 teaspoon salt, ½ teaspoon dry mustard, ½ teaspoon pepper
Slowly add the milk, stirring constantly to prevent lumps. Continue cooking over medium heat until the mixture thickens into a smooth sauce, about 5 to 7 minutes. Add 1 ½ cups of the shredded cheese and stir until it melts completely and the sauce is smooth.
1 1/2 cups shredded cheese
1 ½ cups milk
Add the diced ham, cooked potatoes, and chopped parsley. Stir gently to combine, being careful not to break up the potatoes too much.
½ pound ham, 6 cups diced cooked potatoes, ¼ cup fresh parsley
Spoon the mixture into the prepared baking dish and spread it out evenly. Sprinkle the remaining ½ cup of cheese over the top.
1/2 cup shredded cheese
Bake for 30 minutes, or until the casserole is bubbly and the cheese is melted and lightly golden. Let the casserole sit for a few minutes before serving. Garnish with extra parsley, if desired.