Preheat the oven to 170C/325F/Gas 3.
Slash the skin of the sea bass fillets and place in the fridge.
Cut the fennel lengthways into quarters. Heat the olive oil in a heavy-based ovenproof pan and add the fennel. Cook the fennel over a medium heat, turning frequently, for 20 minutes or until starting to brown.
Add the tomatoes, garlic, boiling water, wine, oregano and some black pepper. Bring to the boil, cover and cook in the oven for 1½ hours.
Remove the pan from the oven and add the olives, balsamic vinegar and basil leaves. Place on a low heat, season and simmer gently.
Take the sea bass from the fridge, rub the surface with the olive oil and season with salt and pepper. Heat a large frying pan. Add the sea bass fillets, skin side down and cook for 3-4 minutes until the skin is crisp. Turn the fillets over and cook for a further minute or until the fish is just cooked through.
Place a spoonful of the fennel and tomatoes into the centre of each plate and top each with a piece of sea bass.