Preheat the oven to 200°C, fan 180°C, gas mark 6. Cook the red lentils following pack instructions.
Meanwhile, mix 2 tablespoons of the olive oil with the curry powder and rub over the fish fillets. Place on a baking tray and bake for 20-25 minutes or until cooked through.
Heat the remaining olive oil in a pan. Add the onion and garlic and cook until tender. Drain the lentils, then add to the pan, along with the tomatoes. Warm through, then stir in the coriander.
Serve the spiced fish with the red lentil mixture on the side.