Preheat the oven to 180°C.
Whisk egg whites to stiff peaks, then fold in lime zest, grapefruit zest, and sea salt.
Spread half of the salt mixture into a large baking dish.
Layer greaseproof paper and sea bream fillets separated by the fish skeleton, ending with salt mixture.
Press the salt crust down firmly to enclose the fish completely.
Bake for 15-16 minutes and rest for several minutes without touching the crust.
Pan-fry sliced fennel in butter and oil until golden and tender.
Deglaze the fennel pan with grapefruit juice.
Whisk olive oil, citrus juices, reduced balsamic, pan juices, and chervil into a dressing.
Break open the salt crust, remove the fillets, and serve over the fennel with dressing.