Boil the potatoes until very tender, about 20 mins. Meanwhile, make the skirlie. Heat the butter in a frying pan, add the onion and cook for 10 mins until golden and soft. Add the oatmeal and stir until the butter is absorbed and the mix looks dry. Continue cooking until the skirlie is toasted, stirring (skirling round the pan) all the time for 5 mins, but do not let it catch and burn. Remove and keep warm.
Drain the potatoes, return to the hot pan and mash really well. Beat in the cream, parsley and skirlie, then season to taste. Serve immediately before the skirlie softens.