Place the 3 quarters of the cod in a large food processor and blitz until smooth. Retain the meatiest quarter of the cod and give it a small dice.
Transfer the puree and diced cod to a large bowl. Add the egg yolk, half of the salt and 1 tablespoon of the cornstarch. Stir to combine.
Place the cod mixture in a large piping bag fitted with a 1" round piping tip (I used an Ateco 808 tip). Pipe the cod mixture onto a baking sheet lined with parchment paper in 2 ½" long logs. You should have around 12. Transfer the baking sheet to the freezer and freeze for 1 hour.
Once the hour has passed, whisk to combine the flour and remaining cornstarch in a large bowl. Whisk to combine the panko and the remaining salt and pour the mixture into a large pie plate.
Working with one stick at a time, roll the fish sticks in the flour/cornstarch mixture, followed by the beaten eggs, followed by the panko mixture. Return the breaded fish sticks to the baking sheet and continue until you run out of sticks.
Heat 1 ½" of neutral oil (I used canola oil) in a large skillet until small bubbles break the surface. Working in batches, fry the fish sticks until golden on both sides - about 2-3 minutes per side.
Transfer the fried fish sticks to a cooling rack over a baking sheet and sprinkle with finishing salt. Serve immediately with oven fries, peas, and the Preserved Lemon Tartar Sauce on the side for dipping.