Instructions

1. Place the potatoes in a pot and cover with cold water. Set them over high heat and bring to a boil. Once the water is at a rolling boil, reduce the heat and simmer until the potatoes are tender, 20-25 minutes.
2. Drain the potatoes and return them to the pot. Stir in the warm milk, butter, salt and pepper. Mash until the potatoes are creamy, adding more milk if necessary. Set aside.
3. Preheat the oven to 400F. In a 10-inch cast-iron skillet, heat up the oil on medium-high heat. Add the onions, carrots and celery, then saute until the vegetables have softened and are beginning to brown, 10-12 minutes. Stir in the garlic and cook for an additional 3-4 minutes.
4. Add the ground beef, salt and pepper and brown the meat, using a wooden spoon to break it into crumbles. Stir in the flour and cook for another 4-5 minutes, until the flour has cooked.
5. Stir in the tomato paste, Worcestershire sauce, rosemary, thyme, red wine or beer (if using) and chicken stock. Bring to a boil, then reduce the heat and simmer, stirring occasionally, until the sauce has thickened to a gravy, about 10-15 minutes. Remove from heat.
6. Stir the frozen peas into the filling and spread it in an even layer in the skillet. Top with the mashed sweet potatoes, creating an even layer of potatoes and smoothing the top. Bake for 25-30 minutes, until the potatoes have just begun to brown. Let sit for 15 minutes before serving.