Peel potatoes and cut into cubes. Place in cold salted water and bring to a boil, reduce heat to low boil until potatoes are fork tender, about 15 minutes. Drain potatoes and mash with milk and 2 tablespoons of the butter until smooth. Season with salt and pepper to taste and set aside. Brown ground beef in a large saucepan over medium-high heat. Remove ground beef from pan and drain off excess fat, reserving 3 tablespoons of fat. Put reserved fat in saucepan add onion, carrot, and celery. Sweat for 5 minutes until onion becomes translucent, add mushrooms, and frozen peas saute for 5 more minutes. Return ground beef to pan along with parsley, thyme, Worcestershire, and nutmeg, simmer for 1 to 2 minutes. Add brown gravy to pan simmer 1 to 2 minutes, season to taste with salt and pepper. Remove from heat and let cool. Preheat oven to 400 degrees. Layer bottom of a 3-liter casserole dish with beef mixture, top with mashed potatoes making uneven peaks with a fork. Cut remaining butter into small cubes and sprinkle over mashed potatoes. Bake for 30 to 40 minutes until mashed potatoes have turned golden brown, remove from oven and let cool 5 minutes.

The recipes for this program, which were provided by contributors and guests who may not be professional chefs, have not been tested in the Food Network's kitchens. Therefore, the Food Network cannot attest to the accuracy of any of the recipes.