In the pitcher of an electric blender, combine rosemary, garlic, orange zest, olive oil, salt, and pepper.
Purée.
Coat pork loin on both sides with mixture.
Seal in an airtight bag, making sure to remove any air.
Place in refrigerator for 2-3 days.
When ready to cook, heat oven to 375℉/80℃.
While oven is heating, set pork loin out of refrigerator.
Place on broiler pan or roasting rack, fat side up.
Roast for about an hour, depending on desired degree of doneness.
Allow to stand for at least 10 minutes before slicing.