Mix the sour cream with the oil and add the mashed banana. Season with salt, pepper and, if the banana is not sweetened enough, with honey.
Cut the fennel into fine strips and chop the fennel green. Finely dice the apples and mix with the fennel and lemon juice. Then fillet the oranges and add them.
Mix with the marinade just before serving and sprinkle with the chopped fennel greens and nuts.