In a 10-inch cast-iron skillet over medium heat, saute the onions until translucent.
Add the carrots and garlic. Saute for 4-5 minutes.
Pour in 1/2 cup beef stock and simmer until most of the stock is absorbed.
Add the lamb, break apart with your spatula and cook until the lamb is no longer pink.
Add the Worcestershire Sauce, tomato paste, rosemary, thyme, nutmeg, and red pepper flakes. Stir to combine and bring to a simmer.
Sprinkle with the flour, mix well, and cook for 2-3 minutes.
Add the remaining 1 cup of beef stock and bring to a boil. Cook for 2-3 minutes.
Stir in the peas and corn.
Remove from the heat and spread the mashed potatoes evenly over the top.
Place skillet on a baking sheet and bake in a preheated 375-degree oven for 45 minutes until bubbly and browned.
Let rest for 15 minutes before serving.