You can use leftover roast chicken for this dish or alternatively roast some chicken breasts or thighs in a preheated oven at 180°C / 350°F / gas mark 4 for 15-18 minutes. Cool and cut into pieces as above
There is a lot of tarragon in this dish which is fabulous and do try and make it with this amount of tarragon if you can but if you are short mix with handfuls of chopped parsley, mint, chives - any soft herb you have really. The sauce takes on a bit of a different flavour but still herby and delicious.
To make breadcrumbs get some slightly stale bread and whizz in a Magimix or with a hand held blender, you can also use shop bought or Panko breadcrumbs.