1. Sprinkle the fish with salt and chill for about half an hour, to firm up. Rinse and pat dry. Cut the fish into 3-4cm wide strips, they should be roughly the size of fish fingers.
2. Heat up 2 tbsp. of oil in a frying pan and add the panko breadcrumbs with the thyme leaves. Toss and stir for about 5 minutes until they are golden and toasted. Tip them out into a large shallow bowl or a plate.
3. In another bowl beat the egg with the mustard. Prepare the third bowl or plate with the flour, for dredging.
4. Using your hands and/or two forks, dredge each piece of fish through the flour to cover it completely, then roll it about in the egg mix to cover, and finally toss around in the breadcrumbs. Place the crumbed fish on a plate lined with paper towels and chill until ready to cook.
5. Preheat the oven to 240C/500F/gas 9. Prepare two baking trays and an oven-proof wire rack to place over one of the trays, to cook the fish on. Put the other baking tray, for the chips, in the lower part of the oven to heat up.
6. Prepare the chips: wash the potatoes, leave the skin on and cut them into 1cm thick sticks. Toss with about 2 tbsp. of oil in a bowl and season with salt and pepper. When the tray is really hot, tip the chips onto it, ideally in a single layer, and bake for 15-20 minutes, tossing them halfway through, until slightly golden and starting to crisp up.
7. Oil the wire rack for the fish, place the fish on the rack and bake in the upper part of the oven for 15 minutes. It will colour only slightly in places. By that time the potatoes should also be ready and crisp, so serve immediately, with cooked green peas, mayo, ketchup or tartare sauce on the side.