ready in 1 hr 20 min.
Preheat the oven to 230°C (210°C in a fan oven), 425°F, gas 7.
Drizzle the lemon juice over the inside of the fish. Season with ground black pepper and stuff with tarragon.
Whisk the egg whites and stir in the salt. If necessary, add enough cold water for the mixture to be workable, but not too moist (max. 200 ml). Put around half the salt mixture in an oven-proof dish and place the fish in the centre. Cover well with the rest of the salt, pressing firmly over the fish, and cook in the oven for around 50 minutes.
Remove from the oven, crack the salt crust (ideally slice around the edge with a bread knife), remove the fish and serve, garnished with tarragon if desired.