Heat the oil in a pressure cooker.
Add the dry spices and toast for 1 minute. Add the shallots, garlic and lemongrass and toast for 2 minutes. Add the tomato purée and cook for 2 minutes. Add the crab shells followed by the water and vanilla pod. Place the lid and bring up to full pressure.
Cook for 35 minutes. Remove from the heat and allow to cool in the pressure cooker. Strai