Blend scallops, egg yolk, cream, and brown crab meat until smooth.
Transfer the mixture to a bowl suspended over ice.
Fold in ginger, white crab meat, lemon juice, and seasonings.
Heat olive oil to 80°C, infuse with basil, blend, and strain.
Boil basil leaves in salted water for 2 minutes and refresh in ice water.
Pat basil dry, blend into a smooth purée, and strain.
Mix basil purée, oil, eggs, flour, salt, and water into a dough and chill for 30 minutes.
Fry shallots, ginger, and lemongrass in butter for 5 minutes.
Add peppercorns and coriander seeds and fry for 2 minutes.
Add brown crab meat and carcasses and fry for 5 minutes.
Simmer with fish stock for 20 minutes and strain into a clean pan.
Whisk in butter, lemon juice, and seasonings to finish the sauce.
Divide the chilled crab mousse into four portions.
Roll pasta thinly and cut into 10cm circles.
Fill circles with mousse, seal into semi-circles, and join ends around a finger.
Boil tortellini in salted water for 5 minutes and drain.
Serve tortellini topped with sauce, grapefruit segments, cress, and basil oil.