Using the same pan with the lamb fat fry the garlic and onions till translucent but not brown.. Add in white wine. Let it boil down to half.
Next add in the lamb stock and let it boil down to half.
Turn off the heat and add in the Dijon mustard and thyme. Then season with salt and pepper and a tbsp of double cream.
Lastly add in the butter and any juice from the resting rack of lamb. The sauce is now ready.
Carve the lamb and serve with the boiled broccoli spears.