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.
Reduce the heat to medium and add the onions and peppers.
Sauté until the onion is translucent.
Add the garlic and sauté 1 or 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 to 8 minutes.
Add the fish fillets, cover and continue to simmer until the fish is cooked through, from 5 to 8 minutes more.
Serve with rice, ugali, boiled potatoes, boiled cassava, or chapati.