Preheat your oven to 375°F. While it heats, prepare all vegetables: peel and cut potatoes into uniform 2-inch chunks, peel and cut carrots into 1/2-inch coins, finely dice the onion into 1/4-inch pieces, core and shred the cabbage into 1/4-inch ribbons, and measure out the peas and parsley. This prep work ensures you won't scramble during cooking and allows you to focus on building flavors.

Heat 1.5 teaspoons olive oil in a large skillet over medium-high heat. Add the ground lamb with 2.5 teaspoons salt and brown for 7-9 minutes, breaking it apart as it cooks, until no pink remains. Transfer the cooked lamb to a bowl, leaving about 1 tablespoon of fat in the pan. Add the diced onion and carrot coins with 0.75 teaspoon salt and sauté for 6-8 minutes until the vegetables soften and begin to caramelize slightly.

Stir the tomato paste into the softened vegetables and cook for 30 seconds to deepen its flavor. Sprinkle the flour over the mixture and stir constantly for 1 minute to create a light roux that will thicken your sauce. Return the cooked lamb and its juices to the pan, then add the Worcestershire sauce, chicken broth, and 0.5 teaspoon pepper. Bring to a boil and stir for 1-2 minutes until the sauce thickens enough to coat the back of a spoon. Remove from heat and fold in the peas and parsley. Transfer this filling to a 9x13-inch baking dish (or similar size), spreading it evenly.

While the lamb filling cooks, bring a large pot of salted water (1 tablespoon salt) to a boil and add the cut potatoes. Boil for 12-15 minutes until fork-tender. Meanwhile, heat 1 tablespoon olive oil in another skillet over medium-high heat, add the shredded cabbage with 0.75 teaspoon salt, and cook for 7-9 minutes, stirring occasionally, until the cabbage is tender with some browned edges. I find that letting the cabbage get a little color adds nice depth to the colcannon. Drain the potatoes well when ready.

Combine the drained potatoes from Step 4 with the cooked cabbage from Step 4 in a large bowl. Add the whole milk and 5 tablespoons butter, then mash until creamy and well combined—I prefer Kerrygold unsalted butter here for its richer, more luxurious flavor. Season with salt and pepper to taste, then add the nutmeg for a subtle warmth. The colcannon should be creamy but still hold some texture; avoid overmixing.

Spread the colcannon topping from Step 5 evenly over the lamb filling in the baking dish, smoothing the top with the back of a spoon. Bake at 375°F for 30-35 minutes until the top is golden and the filling is bubbling at the edges. Remove from the oven and let rest for 10 minutes before serving—this resting time allows the layers to set slightly so you get clean, defined portions.