Place the potatoes in a medium sized saucepan, cover with cold water, add a good bit of salt, loosely cover and bring to the boil. Reduce heat and simmer until potatoes are tender, about 15-20 minutes. About 7 minutes before the potatoes are cooked, put the leeks in a steamer and steam over the potatoes, uncovered, for about 6-7 minutes until tender. Transfer the leeks to a clean tea towel and pat dry. Drain potatoes, return saucepan to a very low heat to thoroughly dry them out. Remove from the heat and mash with a potato masher. Add 1/4 cup of butter, the mustard and the steamed leeks and beat all the ingredients together until you have created a light, fluffy mass. Season with salt and pepper to taste, cover with a clean tea towel and a lid and set to one side while you prepare the cheese sauce.
Preheat oven to 400°F Melt the remaining 1/4 cup of butter in a saucepan over low heat. Whisk in the flour and let cook for a minute or two; remove from heat if mixture begins to colour. Add milk gradually, whisking in as you do so. Increase heat and cook until mixture comes to a gently boil and thickens. If too thick, add a little additional milk. Add all but 2 tablespoon of the cheese and stir in well to melt. Season with salt and pepper to taste.
Lightly butter a shallow gratin dish. Spoon the potato mixture around the outside. Fill the centre with the quartered eggs. Cover all with the cheese sauce and use rest of the cheese for the top. Bake for 20-25 minutes until golden brown and heated through. 4 serving. Serve with a traditional farmhouse dry cider.
NOTE: Make sure to hard boil the eggs correctly so their yolks are not discoloured: place in cold water, bring to a rolling boil, let boil for just a minute, remove from the heat, cover and let stand 15-17 minutes. Run cold water and let sit in the cold water for 5 minutes before shelling and using.