Creamy and comforting, this make-ahead casserole is easy to transport and guaranteed to be a hit at holiday get-togethers.
Preheat oven to 450 degrees F. Place potatoes in a large pot and cover with salted water. Bring potatoes to a boil, and then reduce the heat to a simmer. Cook until they are fork tender, about 15 minutes. While potatoes are cooking, heat olive oil in a heavy oven-safe pot over medium. Add carrots, celery, onion, thyme, rosemary and nutmeg and sauté for about 5-7 minutes. Season with salt and pepper. Add the ground beef and lamb and cook, stirring occasionally for about 10 minutes. Then add the flour and tomato paste and stir, cooking for 1-2 minutes more. Add the beer and Worcestershire and bring to a boil, then simmer for a few minutes more. Add the peas and stir. Set meat mixture aside.
Once potatoes are cooked, drain them and then return to the pan. Add the milk and cheese and mash until smooth. Season to taste with salt and pepper.
Pour meat mixture into a 9- by 13-inch baking dish. Using a rubber spatula, spread potatoes evenly on top (Tip: use the backside of a fork to make your potatoes decorative). Sprinkle the top with a little bit more cheese and bake for about 20 minutes.
Swap: This dish is very versatile. Try ground turkey instead of beef and lamb, or try sweet potatoes instead of white potatoes. You can also top this dish with any cheese you prefer.
Time-saving tip: If you don't feel like making your own, buy store-bought mashed potatoes to save time!