Heat the oil over medium-high flame in a large skillet or pot. Season the fish with salt and pepper. Sear the fish fillets on both sides and remove to a plate. Do not cook through.
Reduce the heat to medium and add the onions and peppers. Sauté until the onion is translucent. Add the garlic and sauté 1-2 minutes more.
Add the tomatoes, coconut milk, garam masala or curry powder, tamarind paste or lemon juice, salt and pepper. Bring to a boil, then reduce heat to low and simmer for 6-8 minutes.
Add the fish fillets, cover and continue to simmer until the fish is cooked through, 5-10 minutes. Serve with rice, ugali, boiled potatoes, boiled cassava or chapatti.
Variations: Mtuzi wa Kamba (Shrimp in coconut curry): substitute shrimp for the fish. There is no need to sear the shrimp initially.