Cook the potatoes in boiling water for 10 to 15 minutes until tender, drain and return to the pan. Mash with the milk.
Meanwhile, heat 1 tablespoon of the oil in a frying pan and fry the onions, covered, for 7 to 8 minutes, or until golden and caramelised. Stir into the mash and season well.
Heat the remaining oil in the same frying pan and fry the sausages for 3 minutes, then add the mushrooms and fry for a further 3 minutes.
Blend the gravy granules with 200ml boiling water and add to the pan along with the mustard. Bring to the boil, stirring. Transfer to an ovenproof serving dish and top with the mash.
Bake for 20 minutes until golden.
Rudolph is a striking, British grown, maincrop potato variety with a vibrant red skin and contrasting snowy white flesh. It has fantastic eating qualities – naturally sweet in flavour with a smooth and creamy texture.