Season the chicken breast fillets with salt and pepper.
Beat the egg white for 2-3 minutes until stiff peaks form.
Fold the grainier mustard, mustard powder, chives, and breadcrumbs into the beaten egg white.
Place the chicken breast fillets in a baking dish and brush the mustard mixture over the top and sides.
Bake in a preheated oven at 200°C (392°F) for 20 minutes until the chicken reaches an internal temperature of 74°C (165°F). Cooking time may vary depending on the thickness of the fillets.