Preheat the oven to 200°C.
Cut a cardboard fish shape with a 2.5cm border, wrap in foil, and place the fish on top.
Whisk egg whites to soft peaks, then whisk in salt until stiff peaks form.
Spread the egg-white mixture over the fish to create an airtight seal.
Bake the sea bass for 20 minutes until the internal temperature is hot.
Fill the squash halves with shallot and garlic, drizzle with oil, and season.
Roast the squash for 25 to 30 minutes until tender.
Blend the roasted squash flesh in a food processor until smooth.
Melt 50g of butter and fry the carrot, mushroom, and banana shallot until softened.
Add 100ml of Champagne and simmer until the liquid is syrupy.
Add the fish stock and reduce the liquid by half.
Add the cream and reduce the liquid by half again.
Strain the sauce, whisk in the remaining butter and lemon juice, and stir in the remaining Champagne.
Crack the salt crust, remove the sea bass fillets, and discard bones.
Plate the squash purée, top with fish, and spoon over the sauce.