Step 1: Put the potatoes into a large saucepan, cover with water and add 1 teaspoon of salt. Boil for 12–15 minutes until the potatoes are tender. Drain well, then tip back into the saucepan over a low heat to steam-dry for a few minutes.
Step 2: Preheat the oven to 190C/170C Fan/Gas 5. Grease a deep baking dish with butter.
Step 3: Mash the potatoes using a masher or ricer, then mix in the butter, milk and mustard. Add two-thirds of the cheese, the spring onions and the egg and briefly mix together. Taste and season with salt and pepper.
Step 4: Spoon the potatoes into the baking dish. You can leave the top craggy, or smooth with a spatula then run a fork over to scruff up the surface. Sprinkle the remaining cheese over the top.
Step 5: Bake for 35–40 minutes until the top is crisp and well browned. Serve with a colourful salad, pickle and mustard, or steamed green vegetables.