Season the frenched lamb rack on all sides with salt and pepper. Place the lamb rack in a sous-vide bag and seal it properly.
Plunge the bag in a water bath at 60°C to 62.5°C for 50 minutes.
Allow the rack of lamb to cool, then sear the rack of lamb in olive oil over medium heat, fat cap side down first to melt the lamb fat for 4-5 minutes.
Add a knob of butter and move the lamb around to ensure that it is evenly seared. Baste with the butter on each side for 4-5 minutes.
Rest the lamb on a wire rack with a catch pan underneath. Carve the rack of lamb and serve with the jus on the side.