Pre-heat the oven to 350°F.

Heat a large skillet over medium heat. Add the ground beef and cook until no longer pink. Remove browned beef and drain the grease.

Add the butter and onions to the pan and cook until they start to brown. Add the celery and carrots. Season with the salt, dried thyme and freshly ground black pepper. Sauté for a few minutes until the vegetable start to soften. Add the tomato paste and continue to cook for a couple minutes to toast the paste. Stir in the flour until it coats the vegetables, stirring and cooking for another 2 minutes.

Deglaze the pan by pouring in the sherry, bringing to a simmer and scraping up any brown bits on the bottom of the pan. Add the beef stock and Worcestershire sauce and return the browned beef to the pan. Bring the mixture to a boil and then lower the heat and simmer for about 5 minutes until the sauce thickens. Stir in the fresh or frozen peas. Transfer the beef to 2-quart casserole dish.

Top the beef mixture with spoonfuls of the colcannon mashed potatoes. Spread out the potatoes to evenly cover the beef, making sure the potatoes touch the edges of the dish. Make swirls in the mashed potatoes with a spoon or spatula or score the top in a diagonal pattern with a knife. Lightly sprinkle some paprika over the potatoes.

Bake in pre-heated oven for 45 minutes until potatoes are lightly browned on top and sauce is bubbling. Let rest at least 10 minutes before serving.