Fry the chopped onions, fennel and fennel seeds and garlic for 5 minutes until soft and golden. Add 500ml of water, 2 sticks of lemongrass, cilantro, vegetable stock cube and a pinch of saffron.
Simmer on medium heat for 20 minutes. Strain, then add 200ml of tomato passata, quartered potatoes and coconut milk to the strained liquid. Reduce heat to medium, cover and simmer for 45 minutes.
Rinse and pat dry the clams and butterfly the prawns. In another pan, add 1 tsp of olive oil and 0.5 tsp of butter. When hot, add the chopped garlic and prawns and clams.
Fry the prawns for a maximum of 3 - 4 minutes, just allowing it to turn slightly pink. Remove from the pan and set aside.
Cover the pan and continue to cook the clams( vongole) for another 4 -5 minutes until all the clams have opened up. Remove the opened clams and set aside, discarding the liquid and any unopened clams.
Add the cooked clams and prawns to the pot of stew, simmer for another 4 - 5 minutes. Add some chopped parsley before serving.