Remove any silver skin or excess connective tissue from the loin, if not already done by the butcher.
If desired, truss the meat with butchers twine. When I sous vide my loin, I don't truss it.
Salt and pepper the outside of the loin and sear on all sides in a hot skillet over medium-high heat, about 1 minute per side. This will set the meat fibers, helping to lock in juices as well as giving the beef extra flavor as it cooks.
Allow the meat to cool for a few minutes before placing it in a vac and seal bag or gallon-sized freezer-safe bag, sprinkling with rosemary.
Using the water displacement method, slowly lower the bag into the preheated water bath and secure the edge of the bag to the side of your cooking vessel.
Cook for 1 hour per inch of thickness of meat, at the thickest point. Example: 3 inch at the thickest = 3 hours cooking.
Preheat the grill to high heat.
Remove from the water bath, drain the juices in the bag in a separate bowl and save them to add to a sauce, if making one.
Pat the meat dry with paper towels and apply a coating of herbs and mayonnaise.
Sear the loin on the grill for 30 seconds to 1 minute per side. The meat should easily pull away from the grill when properly seared.
Remove from the grill and slice into 1 inch slices. No need to allow the meat to rest with the sous vide method of cooking.
Why is mayonnaise used as a coating for searing?
Mayonnaise is the emulsification of eggs and oil. The protein of the eggs and nonstick properties of the oil will give the meat caramelization along with easy release from a super hot cooking surface. (Think 500-700 degrees) The mayo quickly sears and caramelizes while adhering to the protein of the beef.I added garlic powder as well as dried rosemary and salt and pepper to my mayo coating to give the beef a delicious herbed crust.