1. Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius).
2. In the bowl of a food processor, pulse four of the mushrooms for a few seconds or until finely minced (you can also finely chop by hand).
3. Combine the minced mushrooms with the beef, grated potato, grated onion, walnuts, egg, parsley, salt, and pepper. Lightly mix by hand until fully combined.
If you have extra meat, this mixture makes delicious meatballs as well! Roll into balls and sear in a skillet, then add to the sauce of your choice and simmer until cooked through.