Put the oil in a large, deep frying pan and heat gently. Add the onion, garlic, celery, red capsicum, chilli 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 jug or bowl, then set aside. Bring a kettle of water to the 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 1 litre with boiling water, pour into the pan and stir well. Bring to the 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 prawns. Partly cover the pan again and cook for a further 5 minutes or until the prawns 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.