For the fish curry, coarsely chop the onion. Roughly chop the tomato, heat the oil and fry the onion in it until hot. Add coconut milk, oyster sauce, curry paste, salt, pepper and coriander and mix everything well.
Add tomatoes and fish fillet. Simmer gently on low heat for about 15 minutes. Serve the fish curry with chili peppers, basmati rice or jasmine rice.