1 Peel and finely chop onion and garlic. Rinse lemongrass, pat dry and peel outer layers. Cut inner part into slices.
2 Heat 1 tablespoon oil in a small pot over medium heat. Sauté onion, garlic and lemongrass until softened and fragrant, 3-4 minutes.
3 Add curry paste. Cook, stirring, until incorporated, about 3 minutes.
4 Add coconut milk, coconut water and fish stock. Bring to a boil, then simmer over low heat for 20 minutes.
5 Pour coconut-milk mixture through a sieve and set aside.
6 Meanwhile, rinse romanesco, pat dry and cut into small florets. Cook in a pot of boiling salted water until crisp-tender, about 4 minutes. Drain and rinse with cold water to stop the cooking.
7 Rinse tomatoes and drain. Rinse bell pepper, wipe dry, cut into quarters and remove seeds. Cut pepper into 1 cm (approximately 1/2-inch) pieces.
8 Heat remaining oil in a shallow pan or wok. Sauté bell pepper until crisp-tender, about 3 minutes.
9 Add romanesco and tomatoes to pan.
10 Add the reserved coconut-milk mixture to the pan and bring to a boil. Add fish sauce and cook 3 minutes more.
11 Rinse fish, pat dry and cut into large pieces. Season with salt and pepper. Lay fish on top of the vegetables. Cover and cook over low heat until fish is opaque throughout, 7-8 minutes.
12 Rinse cilantro, shake dry, pluck leaves and finely chop. Sprinkle over curry just before serving, with rice, if desired.