Heat the oven to 200 °C / 390 °F.
Combine the mustard, garlic, onion, rosemary, sage, thyme, lemon and orange zest, sea salt, pepper, and olive oil in a bowl and mix well.
Place the turkey legs in a roasting pan and pour the marinade over them. Massage it into every fold and crease, wearing gloves if you prefer.
Tuck the dates and prunes around the legs. Combine the lemon and orange juice, wine, and stock in a bowl, then pour over the turkey and fruit.
Cover with parchment paper and aluminum foil and roast for 2½ to 3 hours.
Uncover and continue roasting until the skin is crisp and deeply golden, about 25 minutes.
Remove from the oven, cover tightly with foil, and rest for 10 minutes before serving.