Heat the sesame and vegetable oils together in a large flameproof casserole, add the shallots and garlic and fry gently for 2 minutes or until softened but not browned.
Add the onion, coconut water, rice wine vinegar, lemon grass, lime leaves, chillies, stock and sugar to the casserole and bring to the boil. Boil for 2 minutes, then reduce the heat and add the tomatoes, fish sauce and tomato purée and cook for 5 minutes.
Discard any clams that don't shut when tapped, then add them with the prawns, squid rings and mushrooms to the casserole and simmer gently for 5–6 minutes or until the prawns turn pink, the squid are cooked through and the clams have opened. Discard any clams that remain closed. Stir in the basil leaves if liked.
Serve the hotpot immediately with rice noodles.