Combine all of the ingredients for the tartare sauce in a small mixing bowl. Season to taste and adjust with sugar to sweeten and balance out the acidity if desired. Cover and chill in the fridge until needed.
Sprinkle both sides of the fillets with 1/4 tsp salt and leave to rest for 10 or so minutes.
Meanwhile, combine the remaining 3/4 tsp salt with the rest of the seasoning in a small pot or bowl. Line up 3 large shallow dishes: 1st with flour + half of the seasoning, 2nd with eggs + Dijon beaten until smooth, 3rd with Panko + rest of the seasoning. Place a tray next to the last bowl to rest the breaded fish.
After 10 mins, thoroughly pat the fish dry to remove as much moisture as possible. One by one, dredge each piece through the flour, into the egg, then into the breadcrumbs. Finish by resting on the tray then repeat with the remaining pieces. Work fairly quickly so the batter doesn't go soggy.
Add enough oil to comfortably cover the base of a heavy-based/cast-iron pan. Turn the heat to medium-high and allow the oil to reach 180C/350F (a breadcrumb will rapidly sizzle when it's ready). In two batches, fry the fish on both sides for 2-3mins, or until golden and crisp with the centre piping hot (safe internal temp is 63C/145F). Rest on a wire rack above a tray or kitchen roll to catch excess oil and immediately place the cheese on top to lightly melt.
Stack the sandwiches with your preferred amount of sauce then tuck in and enjoy!