Place potatoes in a large saucepan, cover by 1 inch with water and bring to a boil.

Lower heat to medium-low and simmer until potatoes are tender, about 15 minutes.

Reserve 1 cup of the cooking water and then drain potatoes well.

Transfer to a heatproof bowl and mash with a potato masher until smooth, adding some of the reserved cooking water to make potatoes spreadable. (Reserve 1/2 cup cooking water for later use.)

Meanwhile, bring a medium saucepan of water to a boil.

Add carrots and cook until just tender, 5 to 7 minutes.

Drain well and set aside.

Cook ground lamb in a large skillet over medium heat, stirring occasionally, until almost cooked through but still juicy, 5 to 7 minute.

Add onion and cook, stirring occasionally, until onion is softened and translucent and lamb is cooked through, about 5 minutes more.

Add reserved 1/2 cup cooking water and cook, scraping up any browned bits, until just thickened, about 1 minute more.

Remove from heat and stir in carrots and peas.

Transfer lamb and vegetable mixture to an 8-inch-square baking dish and spread potatoes evenly over the top and serve. (Optional: Preheat the broiler. Place the dish under the broiler until potatoes are just browned, 2 to 4 minutes, watching carefully to ensure they don't burn.)

The pie will keep refrigerated in an airtight container for up to 2 days or in the freezer for up to 2 weeks.

(Suggested age: 12 months and up)