Rinse the lentils in a fine mesh sieve, then place them in jars or bags with water, bay leaves and salt. Set sous vide devise to 190ºF (87ºC) and cook for 90 minutes, or up to 3 hours. Open and let cool. Drain and set aside.
Alternatively, bring lentils, salt, bay leaves to a boil in 4 cups of water, then simmer until tender, 25-30 minutes.
Place hazelnuts on a rimmed baking sheet and roast at 350ºF for 12-15 minutes. Let cool. Rub off most of the skins, then chop roughly with a knife or pulse in a food processor until roughly ground. Set aside.
Remove the duck from its bag, pouring the juices into a small pan. Bring juices to a boil, then reduce over medium heat until you have about ¼ cup.
Reheat duck in a cast iron frying pan, browning and crisping the skin. Remove the skin and crisp it on all sides. Melt any fat under the skin. Reserve ½ cup of the fat.
Remove duck to a cutting board, let cool slightly. Shred with hands.
Chop onion pickles (use fresh mild onions or shallots if you don’t have the onion pickles) into a fine mince. Chop tarragon and parsley, then throw all three into a bowl with the lentils.
Make the vinaigrette: put the vinegar, garlic cloves, capers, mustard, salt and pepper together in a food processor and blitz until smooth. Pour over the lentils, onions and herbs. Warm up the duck fat in the pan. Toss together thoroughly.
Add half of the hazelnuts and the duck meat and skin, and chopped fois gras, if using, and toss again. Top with remaining hazelnuts and a few sprigs of tarragon.