Peel potatoes and cut into slices about 1/2-inch thick.

Place potatoes in a large saucepan and cover with water. Over high heat, boil and cook until the potatoes are tender, about 10 minutes. Drain and set aside.

Preheat oven to 350 degrees.

Prepare cheese sauce: Measure olive oil into a medium-size sauce pan. Heat over medium heat for 1 or 2 minutes. Then, stir in flour. Cook 1 minute. Stir in milk. Cook, stirring constantly, until the sauce thickens and bubbles. Add cheese and mustard and stir until the cheese melts. Remove from heat.

Pour a layer of cheese sauce into the bottom of a large casserole dish. Add a layer of potatoes and then one of ham. Cover with cheese sauce. Continue layering, ending with cheese sauce on top.

Cover with foil and bake for 25-30 minutes. Remove foil and continue baking until top layer of cheese browns slightly, about another 10 minutes.