Heat 1 tablespoon of the oil in a frying pan, add the leeks and season with pepper, then cook over a medium heat for 8–10 minutes until soft. Add the lentils and cook for a further 2–3 minutes. Stir in the horseradish and crème fraîche and continue to cook for 2–3 minutes.
Meanwhile, heat 1 tablespoon of the oil in a frying pan, add the sea bass and cook for 3–4 minutes on each side or until cooked through.
Serve the sea bass with the lentils, drizzled with the remaining oil.