The Best Shepherd's Pie

For the Filling: In a large cast iron pan over medium low heat, heat oil. Add onion; sauté until it begins to soften, about 5 minutes. Add garlic; cook 30 seconds more. Add carrots; cook another 5 minutes. Transfer vegetables to a plate or push off to one side of the pan. 

In the same pan, add lamb, breaking it up with a wooden spoon; season with salt and pepper. Mix lamb with vegetables. Cook, stirring and continuing to break up meat frequently, until lamb is browned, about 10 minutes. 

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

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

For the Topping: In a large pot, add potatoes with enough salted water to cover them by about 1 inch. Bring to a boil; cook 12 to 15 minutes, until potatoes are tender enough to be easily pierced with a fork. Drain well. Add warmed cream, butter and salt; mash using a potato masher or ricer. Place on top of pie filling, spreading to the edges. Leave craggy swirls on top instead of smoothing out. 

Bake for 45 minutes at 375°F until heated through and mashed potatoes on top have nicely browned spots in a few places. (You may want to place a pan underneath in case filling drips out.) Let cool for 10 to 15 minutes before serving.