Heat 1 tbsp oil in a pan and briefly fry 8 basil leaves just for a moment until they crisp up, then lift out of the oil with a slotted spoon and drain on kitchen paper.
Bring a large pan of water to the boil and get a bowl of iced water ready. Blanch the remaining basil leaves for 5 secs. Using a pair of tongs, lift out the leaves and plunge them into iced water so that they cool quickly and keep their colour.
Once cool, lift the leaves out and squeeze to remove any excess water.
Add the blanched basil leaves to a food blender and pour in 60ml of olive oil. to a pourable sauce, adding more oil if needed, then put in a bowl and set aside.
For the ratatouille, halve the pepper, then deseed and slice into thin strips. Cut the courgette into quarters lengthways and slice. Slice the aubergine into thick rounds, then cut each one into strips.
Heat the oil in a saucepan, add the cumin and garlic, and cook for 30 secs. Tip all the veg into the pan, season and sweat for 1 min. Add 100ml water and cook on a high heat for 4 mins. Adjust the seasoning, adding the sugar if needed.
With a sharp knife, score each fish fillet with small slits through the skin at regular intervals.
To cook the fish, heat the remaining 1 tbsp oil in a frying pan until hot. Season the fillets and place, skin-side down, in the pan. Cook until the skin is golden and crisp, and the flesh is changing colour.
Turn the fillets over and cook for 1 min, remove from the heat, then sprinkle over the lemon juice. Divide the ratatouille between the plates and top with the sea bass. Drizzle with basil oil, a little extra virgin olive oil and serve with crisp basil leaves.