Toast Breadcrumbs: Melt 1 heaped tbsp butter in a pan over medium heat and pour in your Panko. Fry for 5mins, or until the breadcrumbs turn golden, stirring & shaking the pan frequently. Place in a dish to cool.
Seasoning: In a small pot combine paprika, salt, onion powder, garlic powder and white pepper. Place to one side.
Prepare Fish: Slice each loin into 6 wedges (12 in total, 4 per sandwich). They will shrink so cut them slightly larger than what you imagine them ending up like. Thoroughly pat the fish dry with paper towels. Quickly move on to the next step as the fish becomes watery the longer it rests (can pat dry again just before dredging if you need)
Dredge: Line up 3 large bowls. First bowl is flour with half of the seasoning mix. The second is 2 beaten eggs. Third is breadcrumbs, parmesan and the rest of your seasoning mix. One by one dip the fish into the flour, then into the egg and finally into the breadcrumbs, ensuring you fully coat at each stage (especially the last).
Cook: Space out on an oiled wire rack above a baking tray (don't cram together or they'll go mushy as they cook). Spray with oil then pop in the oven at 220C/425F for around 12-15mins, or until golden and crispy all over. Allow to rest for a few mins (they crisp as they cool).
Sandwich: Spread one side of each of your slices of bread with butter. Top with fish fingers, then sprinkle with salt and a dash of lemon juice or vinegar. Slather over a good dollop of tartar sauce, then add the second slice of bread, slice in half and enjoy! If you want both sauces, spread the tartar sauce on top of the butter then squeeze the ketchup over the fish fingers.