Grease a 600ml metal mould and warm 100ml water with lemon juice in a saucepan.
Dissolve the squeezed gelatine into the warm liquid and let it cool slightly.
Extract all white meat from the lobster tail and cracked claws.
Blend lobster meat, prawns, and mayonnaise in a food processor while slowly adding the gelatine liquid.
Fold the lobster mixture into whipped cream, transfer to the mould, and chill overnight.
Sauté shallots, garlic, thyme, and bay leaf in butter, then simmer with champagne and sugar.
Reduce the sauce with cream until it reaches single cream consistency and strain through a sieve.
Briefly dip the mould base in hot water to unmould the mousse onto a plate.
Froth the warm champagne sauce with a hand blender and stir in the chives.
Serve mousse slices garnished with caviar and chives, accompanied by the sauce and melba toast.