Stock: sauté half the onion (330 g) and half the garlic (4) without colour, add reserved crab (1/2) and prawn (3) shells, sauté 1–2 min. Add fish frame (1/2) and water (4 l), boil then rapid-simmer 1 hour; strain.
Beurre manié: hand-mix butter (150 g) and flour (200 g) to a paste; set aside.
Bisque: sauté remaining onion (330 g) and garlic (4) until transparent, add chopped prawn meat (3) and crab meat (1/2), sauté 3 min. Add tomato (600 g), saffron (1 g), pastis (250 ml), cream (340 g) and half the salt (15 g); simmer 4 min.
Add strained stock, boil, blitz smooth with a stick blender (thicken with beurre manié if needed).