Cut fish into bite-sized pieces.
Rinse and halve peppers, remove seeds and ribs, and cut into thin strips.
Blanch tomatoes in boiling water for few seconds, then rinse with cold water, peel, quarter and remove seeds.
Peel and finely chop onion.
In a pan, sauté onions and ginger in hot oil. Add pepper strips and stir in curry paste. Add coconut milk and season lightly with salt. Simmer 3-4 minutes. Add fish and season again with salt, pepper and sugar to taste. Stir in tomatoes and cook 3 minutes more.