Using a slicing knife, make an incision parallel to the cutting board into the pork loin (butterfly). The incision should run from end to end lengthwise, and leave about 1/2 inch of the pork loin intact opposite the side of the incision. You should be left with the pork loin opened into about an 8-by-10-inch piece.
In a medium mixing bowl, combine the sausage, olives, dates and shallots. Mix in the cream, breadcrumbs and egg yolk, and finish with the parsley. Season with the za'atar and some salt and pepper.
Stuff the pork loin by spreading the mixture down one side of the opened loin and folding over the other half. You should leave about 1/2 inch of extra space for the ends of the pork to seal again, keeping the filling centered. Cut 8 long pieces of butcher's twine and tie the loin together to form it back to its original shape.
Put the loin on a roasting rack in a pan and season with salt and pepper. Roast until the internal temperature reaches 150 degrees F, 50 to 60 minutes. Rest the meat for 10 minutes before slicing. You should have 8 even slices from the roast.