Preheat oven to 325 degrees F.
Remove lamb from fridge and season both sides with salt & pepper.
Heat 1 tbsp of olive oil in an oven safe casserole dish or cast iron skillet over medium high heat until hot and shimmering. In batches, so as not to overcrowd the lamb, sear one side of the meat undisturbed for 1-2 minutes until browned. Flip meat to the other side and sear for an additional 1-2 minutes until browned. Using a slotted spoon or tongs, remove the seared lamb (it will not be cooked through at this time) to a plate and set aside.
Repeat the process until all of the stew meat has been browned. This will probably take about 2 batches. Set aside.
In the pan with the lamb drippings, add the diced onion and sautee over medium to medium-high heat for about 3 minutes until slightly softened.
Add in the carrots, Worcestershire sauce and 1 tsp fresh thyme leaves. Stir to combine.
Sprinkle in ¼th cup flour and cook over medium to medium-high heat for 1-2 minutes stirring constantly.
Stream in beef broth, stirring constantly, and bring to a boil.
Once a boil has been reached, season with salt and pepper and turn heat off (but leave the pan on the warm burner). Add in the seared lamb and juices and ½ cup of frozen peas. Stir to combine and remove from the heat.
Arrange the potato slices on top of the meat and then use a pastry brush to brush on the melted butter. *Note* If you don't have a pastry brush you can just drizzle the butter over the potatoes. Season the top of the potatoes with salt, pepper and a little fresh thyme. Cover the pan with a lid or some foil and place in the oven.
Bake the hotpot for 1.5 hours at 325 degrees F. Then remove the lid or foil, turn the heat up to 375 degrees F and cook for an additional 25-30 minutes to brown the outside of the potatoes.
If desired, sprinkle some sharp white cheddar on top of the potatoes and place until the broiler for a minute or 2 until melted and golden.
Let the hotpot set for about 5-10 minutes and then serve. The consistency is a bit thicker than a stew but thinner than a casserole. Enjoy!