- Preheat the oven to 200°C, 180°C, Gas Mark 5.
- Prick the potatoes with a skewer and wrap in cling film and microwave for about 10 minutes on HIGH (900W), turning once.
- Slice the cooked jackets in half and scoop out the potato into a large mixing bowl. Brush the inside of the potato skins with the melted butter and pop into the hot oven to crisp up a little.
- Mash the potato with a fork and add the ham, cabbage, mustard, egg yolk, parsley and sour cream. Mix well..
- In a separate bowl whisk together the egg whites until white light and fluffy. Fold into the potato mixture.
- Remove the skins from the oven. Spoon the potato mixture into the skins, top with the grated cheese and return to the oven for a further 8 minutes until golden.
- Serve with a crisp salad.