Meanwhile, heat the butter and heavy cream in a small saucepan. When potatoes are cooked, add melted butter and heavy cream and mash potatoes with a potato masher.
Heat 2 Tbsp. olive oil in a large sauté pan. Add onions, carrots and garlic and sauté until soft.
Add ground lamb and stir until browned. Stir in chicken stock, crushed tomatoes, Worcestershire sauce and thyme leaves. Season with 1 tsp. sea salt and freshly ground black pepper. Simmer for 25 minutes.
Put the meat into a large glass baking dish and cover with mashed potatoes. Sprinkle grated cheese on top.