Preheat the oven to 350 degrees F. Boil the potatoes until very tender. While this occurs, cook the lamb in a pot until no longer pink. Drain fat; add the onion, carrot, garlic, flour and thyme and cook 5 minutes. Add the tomato sauce, stock or wine, corn and peas and heat through; season with salt and pepper. Spoon into a 9x13-inch casserole. When cooked, drain the potatoes and mash. Whip in the butter, buttermilk, 1 cup of the cheese, salt and pepper. Spread over the lamb mixture. Sprinkle with the remaining cheese. Bake 45 minutes, or until golden and bubbly.