Heat the sesame and vegetable oils together in a large pan, add the shallots and garlic and fry gently for 2 minutes or until softened but not browned.
Add the onion, coconut milk, measurement water, vinegar, lemon grass, lime leaves, chilli, stock or water and sugar to the pan, bring to the boil and boil for 2 minutes. Reduce the heat and add the tomatoes, fish sauce and tomato purée and cook for 5 minutes. Stir in the rice noodles.
Add the prawns, squid rings, clams and mushrooms to the hotpot and simmer gently for 5–6 minutes or until the seafood is cooked. Stir in the basil leaves. Serve the hotpot immediately.