A traditional Welsh comfort dish of soft-boiled eggs nestled in creamy leek and mature Cheddar sauce, topped with buttery mashed potatoes and more cheese, then baked until golden and bubbling.
Preheat the oven to 200°C fan / 220°C conventional.
Boil the potatoes in well-salted water for 15–20 minutes, until completely tender. Drain and leave to steam dry for a couple of minutes, then mash with the butter and warm milk until smooth and creamy. Season well.
Bring a separate pan of water to the boil. Carefully lower in the eggs and cook for 6 minutes. Transfer immediately to iced water to stop the cooking, then peel and halve once cool.
Melt the butter in a large frying pan and gently cook the leeks with a pinch of salt for 8–10 minutes, until soft and sweet.
Stir in the flour and cook for 1 minute, then gradually stir in the milk until smooth. Simmer until thickened, then stir through the Cheddar, wholegrain mustard and nutmeg, if using. Season with salt and plenty of black pepper.
Spoon the cheesy leeks into a baking dish and nestle the egg halves into the sauce, cut-side up. Spoon over the buttery mash, rough up the top with a fork and scatter generously with the remaining Cheddar.
Bake for 20–25 minutes, until beautifully golden and bubbling around the edges.