Heat the olive oil in a large cast iron pan over medium low heat.  Add the onions and saute until they begin to soften, about 5 minutes.  Add the garlic and cook for 30 seconds more. 

Add the carrots to the onions and cook for another 5 minutes.  Transfer the vegetables to a plate or just push off to one side of the pan.

In the same pan, add the ground lamb, breaking it up with a wooden spoon.  Season with the salt and pepper, then mix the lamb with the vegetables and cook, stirring and continuing to break up the chunks of meat frequently, until the lamb is browned, about 10 minutes.

To the lamb and vegetable mixture, add the Worcestershire sauce, tomato paste, rosemary, thyme, red pepper flakes, and nutmeg.  Stir to combine.  Sprinkle with the flour, then stir in to the meat mixture until evenly dispersed, cooking for 1-2 minutes.

Add the beef broth and cook for 3-5 minutes until most of the liquid is absorbed. Then stir in the frozen peas and corn.  Remove from heat and let cool while working on the mashed potatoes.  Either leave in the cast iron pan, if it is oven safe, or transfer to a square baking dish.

Place the potatoes in a large pot with enough salted water to cover them by about an inch.  Bring to a boil and cook for 12-15 minutes, until the potatoes are tender enough to be easily pierced with a fork.  Drain well.

Add the warmed cream, butter, and salt to the potatoes and mash using a potato masher or ricer, then use to top the shepherd's pie filling in either the cast iron pan or a square baking dish, spreading to the edges and leaving craggy swirls on top instead of smoothing out.