Bring a large pan of salted water to the boil and cook the potatoes and swede for 15-20 mins until cooked through. Drain, reserving the water, then pour the water back into the pan and bring to the boil again. Meanwhile, mash the potato and swede with half the butter and a good pinch of salt and pepper, then set aside.

Once the water is boiling again, cook the cabbage for 3-4 mins until tender but still a vibrant green. Drain and add to the potato and swede mix.

Heat the oven to 200C/180C fan/gas 4. Heat the remaining butter in a pan over a medium-low heat and fry the onions for 8-10 mins until softened but not golden. Stir into the potato mix, then spoon into an ovenproof dish, top with the grated cheese, if using, then bake for 25-30 mins until the top is golden brown.