First, preheat your oven to 195°C / 175°C fan oven / 380°F / Gas mark 5.
Crack your egg in a bowl, and add the mustard. Whisk to combine the yolk and egg white until it’s a smooth egg mixture.
Add your walnuts to a food processor and blend until you have finely crumbled walnuts. If you don’t have a food processor, you can also use a knife and chop the walnuts finely. Avoid any large chunks. Combine your walnuts, parmesan and black pepper, then add them to a plate.
Slice the chicken breast fillets in two straight down the middle, so that you end up with 4 thin chicken fillets. Use tongs or a fork, and dip one of the chicken fillets into the egg mixture. Make sure to completely coat the chicken with the egg. Then, move the chicken to the plate with the walnuts. Move the chicken around, making sure to completely coat it in the walnut crumble.
Place the chicken fillet on a baking tray, then repeat the process with the other chicken pieces.
If using 2 chicken fillets, you should now have 4 chicken pieces coated in walnuts. Bake the chicken fillets in the oven until cooked through, about 10 minutes. Make sure that the chicken reaches an internal temperature of 75° C / 165° F before eating, as this is the temperature which makes chicken safe for consumption.