Potato, ham and cheese casserole

1. Hear the oven to 350 degrees. Butter a 2-quart casserole dish.

2. Put the ham in a big bowl and add mustard. Stir to distribute the mustard evenly.

3. Melt 2 tablespoons of the butter in a heavy saucepan over medium heat. Add the onion, season with salt and pepper, and cook, stirring occasionally until tender and golden, about 10 minutes. Add the onion to the ham and stir to combine. Add the potatoes and stir gently, so as not to break up too many of them.

4. Return the saucepan to the heat and add 2 more tablespoons of the butter. When the butter begins to foam, whisk in the flour and cook for a minute or two. Slowly add the milk, whisking to prevent lumps. Bring the milk to a simmer, stirring with a wooden spoon to prevent sticking. Let the sauce simmer until lightly thickened, 2 to 3 minutes.

5. Take the sauce from the heat, season with salt and pepper, and then slowly stir in the cheese, a handful at a time, until smooth. Pour the sauce over the ham and potato mixture and stir to combine. Transfer mixture to the casserole dish.

6. Melt the remaining 2 tablespoons butter And toss the bread crumbs with it. Sprinkle the bread crumbs over the casserole and bake until the top is golden and the edges are bubbling, about 45 minutes. Let sit for at least 5 minutes before serving.