1/ Remove foie gras from fridge 1 hour before to soften up. Separate the 2 lobes. Devein. Season with salt (15g/kg) and pepper (5g/kg).
2/ Remove duck breast from fridge. Season with salt (15g/kg) and pepper (5g/kg). Use a sharp knife to remove fat from the meat lengthwise and 2/3 of the width.
3/ Weigh 150g of foie gras. Insert into the duck breast between the fat and meat. Sew the fat and meat together tight enough that the foie gras cannot escape during cooking. Bonus tip: complete steps 1-3 quickly to avoid the foie gras getting too warm and hard to handle.
4/ Bring water to the boil in a large pan then turn down to a simmer. Put the roast in a sous vide bag or make a tight ballotine using cling film. Cook the ballotine in water for 20 minutes.
5/ After 20 minutes, remove roast from bag or cling film (otherwise the fat may harden and leave an unpleasant-looking yellow film) and leave to cool in the fridge.
6/ Cut into 3cm thick slices and serve with some baby leaves and onion chutney.