Preheat oven to 375 degrees. Bring a large pot of salted water to a boil. Add potatoes and cook until tender, about 15-20 minutes. Drain and mash. Place in a large bowl and mix in butter, sour cream, milk, cheese, and salt and pepper to taste. You can add more milk to your desired consistency. This yields about 4 1/2 cups of mashed potatoes. Set aside.
While potatoes are boiling, heat the oil in a large frying pan. Add onion and cook until clear. Add the garlic and cook for 30 seconds. Add ground bison, steak seasoning, and Worcestershire and cook until well browned. Stir in flour and cook 1 minute. Add ketchup, beef broth (or broth of choice), and frozen carrots and peas. Bring to a boil, reduce heat and simmer for 5 minutes.
Spread the ground bison in an even layer on the bottom of a 2 quart casserole dish.
Spoon the mashed potatoes into a large Ziploc bag and cut a large hole in one corner. Pipe mashed potato ghosts and place two peas as eyes. Sprinkle the remaining cheese around the ghosts.
Bake in the preheated oven for 20 minutes, or until golden brown.