Preheat oven to 180ºC. Lightly grease a 7cm deep, 10 x 20cm loaf pan. Place walnuts into non-stick frying pan over high heat. Cook, tossing for 5 minutes until toasted. Remove to a board to cool.
Add the oil, onion and garlic to the frying pan. Reduce heat to medium heat. Cook for 3 minutes until soft. Add mushrooms, increase heat to high and cook 4 minutes until mushrooms are soft and all moisture evaporated. Set aside to cool.
Combine the mushroom mixture, chicken mince, breadcrumbs, parsley, egg, salt and pepper and walnuts in a large bowl. Mix until well combined. Spoon the mixture into the greased pan. Use your hands to press down firmly and smooth the surface. Bake for 40-45 minutes or until light golden and cooked through.
Set aside in the pan for 10 minutes before transferring to a rack.