In a large skillet on medium high heat, add the ground lamb and begin to brown it, breaking it apart with a spoon. Once browned, add in the onion, celery, and garlic. Cook for 5 minutes, or until the onions have become translucent. Add the Worcestershire sauce and mix well.
Add in the tomato paste and combine well. Add in the broth, frozen peas and carrots. Bring the mixture to a simmer, then remove from the heat and set to the side.
Preheat the oven to 400 degrees. Place the potatoes in a medium to large saucepan and cover with water. Salt the water as well! Bring the potatoes to a soft boil and cook for about 15 minutes, or until a fork inserted into a potato comes out easily. Drain the potatoes and return them to the same pot, allowing them to dry out for a minute or two to ensure any extra moisture has evaporated out.
Add in the butter, sour cream, heavy cream, garlic powder and salt and pepper. Mash the potatoes well. Mix in the parmesan cheese and set to the side.
Pour the meat mixture into a 9x11 baking dish and spread it evenly on the bottom. Top with the potato mixture and spread evenly. Using a fork, create marks on top of the potatoes. This gets the top more brown and crunchy. Place the dish onto a baking sheet and bake for 25-30 minutes, or until golden brown. Allow the pan to cool for a few minutes before serving.
Serve alongside a fresh salad, sautéed vegetables or any of your favorite side dishes.