Mix all the vegetables and the chopped herbs in a large roasting tin and season with salt and pepper. Spray with a little olive oil and place in a preheated oven, 180°C (350°F), Gas Mark 4, for 40–45 minutes or until the vegetables are tender.
Put the flour on a plate and season with salt and pepper, then dust the plaice fillets in the flour. Heat the oil in a large, nonstick frying pan over a medium heat, add the fish, skin side down, and fry for 2–3 minutes, then turn the fish over using a fish slice and cook for a further 2–3 minutes or until golden. The flesh should be white and flake easily when pressed in the centre with a knife.
Spoon the vegetables on to serving plates and top with the plaice fillets. Serve immediately, garnished with the shredded basil.