Preheat oven to 425° F
Lightly coat tenderloin all over with olive oil.
Rub 1 to 2 Tablespoons of the za'atar spice blend into all sides of the pork.
Brown tenderloin on all sides, about 2 to 3 minutes per side
Remove skillet from stove and roast in the oven until it reaches an internal temperature of 145°
Place tenderloin on a cutting board and tent with foil. Let rest for 10 minutes.
Lightly fry sage leaves in olive oil until they turn a little crispy, then let cool on paper towel lined plate.
With food processor in motion add olive oil in a steady stream until smooth.
Add water little by little, if necessary, to reach a thinner consistency.
Combine frisée and grapefruit in a salad bowl.
Dress to taste or serve dressing on the side.