Heat the oven to 180 C, Gas 4. Cut the potatoes into rough chunks. Remove any tough outer leaves from the cabbage and finely slice the leaves.
Cook the potatoes in a large pan of simmering water for 10 minutes then add the cabbage and cook for a further 10 minutes. The vegetables should be tender but not overly soft. Add the spring onions for the last minute of cooking. Drain well.
Add the butter and most of the cheese. Mash together well, seasoning with salt and pepper. Pile into a buttered ovenproof dish,scatter with the remaining cheese and bake for 20-30 minutes until lightly browned and steaming hot.