Boil potatoes with skins on until cooked.

Dice the boiled potatoes.

Cube the ham.

Chop the green pepper.

Dice the onion.

Combine ham, green pepper, onion, and potatoes in a casserole dish.

In a saucepan, melt butter over medium heat.

Whisk in flour to make a roux.

Gradually add milk, whisking constantly to prevent lumps.

Cook until the sauce thickens.

Add Velveeta cheese to the sauce.

Stir until cheese is melted and smooth.

Pour the cheese sauce over the ingredients in the casserole dish.

Bake in a preheated oven at 350°F (175°C) for 1 hour.