Melt the butter in a heavy based, circular, oven friendly pan. Add the onions and sweat off for a couple of minutes. Add the ham and stir well for a couple of minutes. Slice the potatoes carefully and add them to the pan with the ham. Season lightly with salt and black pepper and stir gently taking care not to break the potato slices.
Level off the ingredients and place the circular cheese on the top off the potato and ham mix. Stud the cheese with the garlic and rosemary, then drizzle over the white wine.
Dust the dish with the mustard powder and paprika then place into a pre-heated oven at 180c and cook for 15 - 20 minutes.
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