Preheat the oven to 350°F.
Place a casserole dish over medium heat and add the olive oil. When the oil is hot, add the onion, garlic and carrots then cover. Reduce the heat and let sweat for 5 minutes.
Remove the lid and increase the heat to high. Add the ground lamb and cook until browned then stir in the tomato paste, hot stock and frozen peas. Season with salt and black pepper, and simmer over low heat for 15 minutes.
Meanwhile, make the colcannon topping: Place the potatoes, whole and unpeeled, in a saucepan with the largest ones at the bottom and fill the pan halfway with water. Cover the pan and place over high heat. When the water begins to boil, drain off about half of it, leaving just enough for the potatoes to steam. When the potatoes are cooked, about 30-40 minutes depending on their size, drain and peel (hold them in a kitchen towel if they are too hot to handle). Place in a warm bowl.
In a saucepan, heat the butter over low to medium heat until melted then stir in the leeks. Cover and cook for 2-3 minutes. Remove the lid and stir in the milk and nutmeg. Heat until the milk has warmed through.
Mash the potatoes, gradually adding the warm milk and leek mixture. Season to taste with salt and black pepper.
Transfer the lamb mixture to a medium-size pie dish and spoon the colcannon topping over it. Brush the top with melted butter to get a crispy golden finish. Bake in the oven for 25 minutes and serve.