Preheat the oven to 220°C/425°F/gas 7. In a bowl, stir together the Maldon salt and egg whites until they are well combined and have the consistency of wet sand. Take a large baking sheet and spread half of the salt mixture into a rectangle just larger than the megrim sole. Put the fish on top. Halve the lemon crosswise and cut three slices from one half, reserving the remaining half. Insert the slices of lemon and herbs into the cavity of the fish. Pat the remaining salt mixture over the fish to cover it completely (you can leave part of the head clear of salt if you want to use a temperature probe to check when it is ready) and bake it in the middle of the hot oven for 20–25 minutes.
Take it out of the oven and rap all around the edge of the salt crust with the back of a large spoon to loosen it. Then lift the top off. Debone the fish: lift the meat from the fish and put it onto a serving platter. Squeeze the juice from the reserved lemon half over fish and drizzle with a little oil. Scatter a few herbs over the dish and serve immediately.
Tip: If you wish to trim off the salt skin before serving that’s fine, but if you want the wow factor, serve the fish with the salt intact, crack it and do the work at the table. It’s simple.