If fish is fresh, place into freezer for 30 minutes to harden it up (it will be easier to cut). If fish is frozen, thaw it until it's not completely thaw but soft enough to get a knife through it.
Finely chop the fish with a chef's knife.
Add the chopped fish to a mixing bowl along with 3/4 cup of the Panko, 1 egg, mayonnaise, mustard, onion powder, salt and cayenne pepper. Mix to combine.
Form the fish into the shape of a stick or log about 1 1/2 ounces in size and place it onto a sheet pan or platter. The key thing is to try and get the fish to be about the same size for even cooking. I use a kitchen scale to weight out each piece.
Refrigerate for at least 30 minutes. This will make breading the fish easier as the fish won't fall apart in your hands. If you can fit them place them in the freezer to stiffen up, but don't freeze them all the way.
Grab three shallow pans and/or bowl. Add flour to one bowl. Add eggs to another bowl, scramble them. Add remaining Panko bread crumbs to the last bowl.
After 30 mintues, place each stick on at a time into the flour mixture. Coat throughly, then move to the egg mixture, gently remove and let any excess drip off, then gently rolled them into the Panko. Set the finished stick onto a plate and continue until all the fish is coated.
Heat up your oil in a Dutch oven or heavy bottom pan. Use a thermometer to gauge when the oil hits 350 degrees. Regularly check the temperature and adjust your heat up and down accordingly to maintain as close to 350.
Fry the sticks in at least 2 batches so not to overcrowd your pan or reduce the temperature of your oil. Cook for 2 to 3 mintues or until golden brown.
Remove to a sheet pan covered in paper towel, topped with a drying rack inverted to help wick away excess oil. Allow to cool a few minutes before serving.