Finely grate the peel of 1 orange. Peel and fillet the oranges. Peel the onion and cut into very fine rings. Clean the fennel and cut into very fine slices.
Orange mustard dressing: Put the grated orange peel with olive oil, orange juice, mustard, salt and pepper in a glass with a screw cap, put the lid on and shake vigorously. Mix the orange fillets, fennel and onion, arrange on a serving platter. Sprinkle with parsley and cover with dressing.