Cook the potatoes in salted boiling water. Drain, then rinse under cold running water to stop the cooking process and drain again.
Pour 1 tablespoon of the oil into a frying pan over a high heat, add the yellow pepper and fry for 2 minutes until slightly coloured but still crunchy. Add the courgettes and tomatoes and cook until the tomatoes start to break up. Quarter the potatoes lengthways and add to the rest of the vegetables. Finally, stir in the spring onions and basil leaves. Season to taste with salt and pepper.
Place the lemon sole on a baking sheet covered in foil. Season the fish with salt and pepper and drizzle with the remaining oil. Place under a preheated grill and cook for 5–6 minutes on each side. Dot each fish with a little butter and squeeze over some lemon juice. Serve with the warm ratatouille.