Mix together the lemon juice, vinegar, cumin, chilli powder, turmeric and salt in a shallow glass bowl. Add the fish and turn to coat evenly. Cover and chill for 25–30 minutes.
Meanwhile, heat a wok over a high heat and add the oil. When hot, add the onion, garlic, ginger and mustard seeds. Reduce the heat and cook gently for 10 minutes, stirring occasionally.
Add the tomatoes and sugar, bring to the boil, reduce the heat, cover and cook gently for 15–20 minutes, stirring occasionally.
Add the fish and its marinade, stir gently to mix and then cover and simmer gently for 15–20 minutes or until the fish is cooked through and flakes easily.
Garnish with chopped fresh coriander and green chillies and drizzle over some natural yogurt, if desired. Serve with steamed basmati rice, pickles and pappadoms.