Soak bread in lukewarm water. Mince cornichon and pearl onions. Squeeze excess liquid from bread and mix with meat, cornichon, pearl onions and walnuts. Season with mustard, salt and pepper. Knead to combine.
Put paper baking cups in mini muffin tin and spoon the meat mixture into the baking cups to make 24 equally-sized portions.
Bake preheated oven at 180°C (approximately 350°F) for 20-25 minutes. Serve hot or cold.