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Heat oven to 200C/fan 180C/gas 6, then toss the potato chunks with the oil and some seasoning on a baking sheet. Roast for 20-30 mins until golden and crisp on the outside, then leave to cool.
To serve, mix the mayonnaise with the lemon juice. Pile a scrunched up piece of ham on each potato, then top with a blob of the lemony mayo. Arrange on a platter with the watercress, then serve.