Heat oven to 200C/fan 180C/gas 6. Prick the potatoes and bake them for 50 mins-1 hr or until tender. Remove from the oven and turn the temperature down to 180C/fan 160C/gas 4. Cut the potatoes in half and carefully scoop out the potato flesh, leaving the skins intact. Mash the flesh with the butter.
Remove the rind from the camembert and mix the cheese with the mashed potato, ham, crème fraîche, cream or soured cream (if using), the mustard and egg. Mash together and check the seasoning, adding freshly milled black pepper and salt if you want.
Heat the grill. Put the potato skins on a baking sheet and divide the filling between them. Top with the gruyère. Oven-bake for 10 mins, then quickly flash under the hot grill until the tops of the potatoes are golden and bubbling.