Soak the fish pieces in milk for at least 30 minutes.
In a small bowl whisk together the egg and chilli sauce.
In a separate bowl stir together the flour, cornmeal, pepper, mustard and Cajun seasoning.
Dip fish pieces into the dry mixture then into the egg then back into the dry mix. Set on a plate in the refrigerator and chill for about 15 minutes. This will help the batter stick to the fish.
Heat more than enough oil to cover the bottom of a large heavy frypan over medium-high heat. Fry fish pieces for 3 to 4 minutes per side or until golden brown. Depending on how much and what kind of Cajun seasoning you used, some of the crumbing may get darker than the rest.
Drain the fish on paper towels and serve with chilli or tartar sauce.