Make the clarified Sous-jus. This is explained in detail HERE.
Put the butter and the sous-jus in a Ziploc quart bag. It is not necessary to seal the bag–drape the opening over the edge of the vessel and secure it with the lid.
Lower the bag into your sous vide bath set at
165 F/74 C.
Allow 1/2 hour for the butter to become fully melted. Remove from bath and pour butter mixture into a tall, clear, 16 oz./450 ml glass that the stick blender just fits into the bottom of. This is important in order to create the correct amount of localized friction. Lower the stick blender into the glass and let the liquids settle. It is essential that the water based products accumulate in the bottom and the oil hovers above. Slowly begin to pulse the stick blender.
Do not lift it up.
Again, this is essential in order to maximize friction in the bottom of the glass. Continue to pulse, and you will see the liquids begin to emulsify.
Continue pulsing and slowly tilt the stick blender to pull more oil down into the sauce. A little oil will always remain on the top, and it can be stirred in with a spoon. Even at this point, running the stick blender in the top of the sauce may still undo the emulsion!