Season the duck legs with a pinch of salt and pepper, rubbing it into the skin. You'll brown the skin once it is cooked. Vacuum seal the duck legs in one bag.
Place the duck legs into the sous vide machine and ensure they are under the water. Cook for 24 hours. Check once or twice during the cooking time to ensure they are still below the water.
The sauce can be made at any time during the 24 hour cooking time. Chop the onions
In a heavy bottomed pan, add 1 - 2 tbs of oil. Heat gently then add the onions. Cover with a lid and cook slowly until really soft. This will take at least 10 minutes. Check occasionally to ensure the mixture is not burning
Crush the cardamom pods. Easiest way is to place a cooking knife on top and hit the knife with your hand.
Add the star anise, cardamom pods and garlic to the pan. Add half the bottle of cider (250 ml) and allow to cook slowly. The sauce should be just bubbling.
After about 10 minutes, remove the garlic, star anise and cardamom pods and add the rest of the cider. Cook for a further 10 minutes. At this point the sauce will be quite thick. You can add more water for a more runny sauce.
Test for seasoning and add salt or pepper to taste. Set aside. Cool and place in fridge if not using immediately
The apple crisps can be made at any time during the 24 hour cooking time. Preheat the oven to 175ºC. Wash and slice the apples very thinly (it helps to leave the skin on) and place in water to avoid browning whilst you cut the rest.
Remove the cores from each slice retaining as much of the shape as possible. Brush with walnut oil on both sides and place on a tray topped with baking parchment to avoid sticking.
Place in the oven and cook until lightly golden. This will very much depend on your oven - at least 30 minutes. Keep an eye on them as they will suddenly start to burn.
Cool then store in airtight container until required. The slices are not as crisp as a potato crisp
Prepare any accompaniments to the duck such as potatoes or vegetables.
Finish the duck. Take the duck out of the sous vide machine. If you are not ready to serve the duck dunk it in cold water in the bag and store in the fridge. The benefit of this is that any fat will set and make it easier to remove.
When you are ready, gently heat the butter in a frying pan and brown the duck skin. Ensure the meat is hot right through and golden brown. Meanwhile, add any juices from the bag to your sauce and reheat. Re check the seasoning.
Serve the sauce to the side of the duck and add the apple crisps.