Preheat the oven to 200°C, gas mark 6.
Fill the cavity of the fish with lemon slices.
In a medium bowl, combine the egg whites and salt with your hands until a thick paste forms.
Place the fish on a tray lined with baking paper. Mound the salt paste evenly over the entire fish. Press the mixture firmly down to the baking sheet, being careful to seal any cracks.
Bake the fish until the crust is golden brown and very firm, about 25 minutes.
Slide the baking paper and fish onto serving platter or cutting board and crack the crust with a handle of a large knife.
Remove the skin, slide a spoon under the top fillet to separate it from the spine and place it on a serving platter. Turn the fish over and repeat.
To serve, garnish with lemon slices.