In a pan over medium heat, heat oil. Add onions, garlic, ginger, chili peppers and cook until aromatic. Add tomatoes and cook, mashing with back of spoon, for about 2 to 3 minutes or until softened.
Add coconut milk and bring to a simmer. Continue to cook until slightly reduced and thickened. Add curry powder and stir until dissolved. Add fish and cook for about 3 to 5 minutes or until cooked through. Season with salt and pepper to taste. Serve hot.