Colcannon, a classic Irish side dish of mashed potatoes mixed with chopped cabbage, adds intrigue to this otherwise traditional shepherd's pie.
Filling: Preheat oven to 375 degrees. Heat a large skillet over medium-high. Swirl in oil. Add lamb; season with 1 teaspoon salt and cook, breaking up into bite-size pieces and stirring occasionally, until browned in places and just cooked through, 7 to 9 minutes. Transfer to a bowl.
Remove all but 1 tablespoon fat from skillet; return to medium heat. Add onion and carrots; season with 1 teaspoon salt. Cook, stirring occasionally, until vegetables are softened and golden in places, 6 to 8 minutes. Stir in tomato paste; cook 30 seconds. Stir in flour; cook 1 minute. Return lamb and accumulated juices to skillet. Stir in Worcestershire, broth, and 1/4 teaspoon pepper. Bring to a boil, scraping up browned bits from bottom of skillet. Reduce heat to medium-low; simmer until thickened slightly, 1 to 2 minutes. Remove from heat. Stir in peas and parsley. Transfer to a 2-quart baking dish.
Mash: In a large pot, cover potatoes with 1 inch of water; add 1 tablespoon salt. Bring to a boil, then reduce heat to medium and simmer until tender, 12 to 15 minutes; drain. Return pot to medium heat; swirl in oil. Add cabbage and 1/2 teaspoon salt. Cook, stirring occasionally, until collapsed and tender, 7 to 9 minutes. Return potatoes to pot; mash with a potato masher. Add milk and butter, mashing and stirring until mixture is creamy and cabbage is distributed evenly. Season with salt and pepper. Spread mash evenly over lamb mixture; brush top with butter. Place on a rimmed baking sheet lined with parchment.
Bake until golden brown in places and bubbling along edges, 30 to 35 minutes. Let stand 10 minutes before serving. Pie can be refrigerated in an airtight container up to 2 days.
For a creamier mash, press cooked potatoes through a ricer into the pot with cabbage before adding milk and butter.
Traditional shepherd's pie is made with lamb, but you can also substitute ground beef or use a combination of lamb and beef.