Heat a sous vide water bath to 130F degrees.
Add all the ingredients, except the butter and beef stock, in a bowl and mix together.
Rub the mixture all over the lamb and place in a freezer safe bag (for water displacement method) or vacuum sealable bag. Seal the bag. (Be careful using the vacuum sealer as the bones may puncture holes in the bag when sealing).
Cook in the water bath for 2 hours and then remove.
Open the bag and remove the lamb, reserving the bag juice in a small saucepan.
Add the beef stock to the bag juice and simmer for 3-5 minutes, skimming any impurities off the top. Adjust salt to taste and set aside.
Heat a skillet on medium-high heat and add the butter.
Add the lamb and brown on each side, basting with butter until golden brown all around, about 2 minutes total.
Remove and let cool slightly. Slice and serve drizzled with the jus.