Place the potatoes in a large pot of cold, salted water. Bring to a boil and cook until the potatoes are tender, about 15 minutes.
Meanwhile, brown the lamb in a large sauté pan, drain off excess fat, remove the lamb and set aside.
In the same sauté pan, add 2 Tbsp. olive oil. When the oil is hot, add the leeks and cook, stirring frequently, until the leeks begin to soften, about three minutes.
Add the carrot, brussel sprouts, turnip and garlic.
Continue to sauté for two more minutes, then stir in the beef broth, Worcestershire sauce, and fresh thyme. Cover with foil or a lid and simmer until the vegetables are just tender, about 10 minutes.
While the vegetables are cooking, drain the potatoes and return them to the pot. Add the butter, milk, sour cream cheddar cheese. Mash well and season with salt and pepper to taste.
When the vegetables are tender, stir in the lamb.
Pour the lamb mixture into a medium sized casserole dish and spread the potatoes on top.
Bake for 20 to 30 minutes until the filling is bubbling hot and the potatoes have topping begins to brown.
Remove from the oven, let rest for 10 minutes then serve in bowls.