For the pesto: In a food processor fitted with a steel blade, pulse the basil and mint 3 to 4 times or until chopped, then add the pine nuts and salt and pulse an additional 2 to 3 times. Add the garlic and cheese, pulse 2 times. With the food processor running, slowly add the oil to blend, this should take 1 minute. Don't over-pulse or puree; this will result in a brown liquid sauce, not a bright herb pesto.
For the pork: To butterfly the pork, place each loin on a cutting board and slice lengthwise through the center, about 3 inches deep, but not cutting all the way through, until it opens to form a 10 by 10-inch square.
Preheat the oven to 350 degrees F.
Season the inside of both loins with salt and pepper and evenly spread the pesto, leaving a 2-inch border on all sides. Sprinkle with feta and top with spinach. Roll tightly lengthwise. With butchers twine, tie several knots around the stuffed loins, 4 to 5 times, to ensure even cooking.
Heat the oil in a heavy bottomed saute pan over high heat, season the outside of the loins with salt and pepper, and sear on all sides, 2 to 3 minutes per side. Remove from the pan and place the loins on a baking sheet lined with aluminum foil and bake for 20 to 25 minutes, or until the internal temperature reaches 165 degrees F. Allow to rest for 5 to 10 minutes before slicing and serving.