Place the fish fillets in the freezer for 30 minutes or until firm, then finely chop. (If you bought them frozen, thaw for half an hour at room temp before cutting.)
Place fish in a bowl along with 3/4 cup of the panko, 1 egg, the mayonnaise, mustard, onion powder, salt and cayenne pepper. Use your hands or a large spoon to combine.
Shape the fish mixture into 1 1/2 ounce finger-shaped sticks. Set on a sheet pan and refrigerate for 30 minutes.
Beat the remaining 2 eggs in a shallow pie pan. Place the remaining cup of panko in a second pie pan and the flour in a third.
Roll each stick in the flour, then egg, followed by the panko and place back on pan. (If you're not ready to cook, cover with plastic wrap and refrigerate for up to 6 hours.)
Place the oil in in the Dutch oven, attach the thermometer and bring to 350 degrees F over medium heat. Fry the sticks in 2 batches, for 2 to 3 minutes, until golden brown. (The internal temp should read at least 145 degrees F on an instant-read thermometer.) Remove with a spider to an inverted rack set over a paper towel-lined sheet pan until ready to serve with your favorite tartar sauce.