Heat the butter in a pan. Briefly sauté the onion and garlic. Add the lentils, pour in the water, bring to the boil. Simmer the lentils (uncovered) for approx. 15 mins. until just soft. Add the spinach, allow to wilt, season with salt.
Season the fish. Heat the clarified butter in a non-stick frying pan. Fry the fish fillets in batches for approx. 2 mins. on each side, remove. Fry the lemon slices in the same pan for approx. 1 min. on each side, place with the fish.
Pour the wine into the same pan and loosen any bits that have stuck to the bottom, pour in the cream, bring to the boil, stir in the mustard, season with salt. Place the fish and lemon slices in the sauce, heat gently. Serve the fish fillets with the lentils.