1 onion , thinly sliced
3 tablespoons vegetable oil
4 big riped tomatoes, coarsely chopped
2 cloves garlic, crushed
5 teaspoons fish sauce
2 stalks lemongrass, cut into 3-inch pieces and bruised with the back of a knife
1 medium serrano pepper, thinly sliced into rings, or a large pinch of crushed red-pepper flakes
2 cups Chicken Stock,
1 cup water
1/4 kg large prawn, peeled and deveined
1/4 kg skinless red snapper fillets, cut into large chunks
1/4 kg mussels, scrubbed well
1/3 cup coarsely chopped coriander leaves
Cooked basmathi rice, for serving
1/4 cucumber, peeled, seeded, and sliced
Lime halves, for serving

In a medium pan, heat oil over medium heat. Add onion and garlic, and cook, stirring, until translucent but not brown, 3 minutes. add tomatoes, fish sauce, lemongrass, and pepper, and cook for 3 minutes.
Add stock and water. Bring to a boil. Reduce to a simmer, cover, and cook for 18 minutes.
Add prawn, fish, mussels, and the Coriander. Cover, and cook just until mussels open, about 5 minutes. Add some butter to taste if you like. Ladle over rice, and garnish with diced cucumber and lime half.