Preheat the oven to 200ºC/400F/Gas mark 6. With a very sharp knife make small close together slits through the potato as if slicing it finely but without cutting all the way to the bottom so the potato remains whole but with many small cuts along its length.
Rub all over with oil, making sure it gets into the slices in a roasting tin. Season well and roast for 1 -1hr 15 minutes until golden and crisp on the outside and cooked in the middle.
When the potatoes are roasting, make a small amount of simple thick béchamel sauce. In a saucepan, melt the butter. Once melted add in the flour and cook for 1 minute. Add the milk a little at a time, stirring out the lumps before adding more milk. Bring to a simmer for 1 minute, then add the mustard and grated cheese. Stir until everything is melted and combined.
Once the potatoes are roasted, pour over the cheesy béchamel and sprinkle over the diced ham and thyme leaves. Return to the oven or a hot grill until golden and bubbling.
Serve with a little extra black pepper and grated parmesan cheese.
Notes
Most potatoes can be hasselbacked and there doesn’t seem to be a traditional sort – I like a roasting spud like a maris piper as it has a fluffier texture but you can use a more waxy spud or even a new potato.