Put the oil in a large, deep frying pan and heat gently. Add the onion, garlic, celery, red pepper, chili and cumin. Cook, stirring often, for 10-12 minutes until softened. Add the rice and cook, stirring, for 2 minutes.
Drain the tomatoes in a sieve over a heatproof measuring cup or bowl, then set aside. Bring a kettle of water to a boil. Add the tomatoes to the rice, sprinkle the thyme over the top, stir well and reduce the heat a little.
Make up the tomato juice to 4 cups with boiling water, pour into the pan and stir well. Bring to a boil, then reduce the heat, cover the pan with the lid slightly ajar, and simmer gently for 10 minutes.
Season the rice to taste, then place the pieces of fish on top. Continue cooking, partly covered as before, for 5 minutes. Stir the rice carefully and turn the fish over, then add the shrimp. Partly cover the pan again and cook for a further 5 minutes or until the shrimp have turned pink, the fish pieces are cooked, and the rice is tender. The dish should be moist, not dry.
Remove from the heat, cover tightly and leave for 5 minutes. Scatter the celery leaves and parsley over the top and serve with lemon wedges to squeeze over.
Each serving: 27 g protein, 16 g fat of which 2 g saturates, 64 g carbohydrate, 8 g fibre, 378 Calories