Marinate the Pork: In a large bowl or zip top bag, combine the lemon zest, lemon juice, 3 cloves minced garlic, paprika, oregano, ½ cup olive oil, salt, and pepper. Add the pork tenderloin, turning to coat. Seal or cover and refrigerate for at least 30 minutes, or up to 4 hours.
Make the Chimichurri: While the pork marinates, prepare the sauce. In a medium bowl, combine ½ cup olive oil, red wine vinegar, chopped parsley, chopped green onions, 2 cloves minced garlic, red pepper flakes, salt, and pepper. Stir well. Let sit at room temperature while you grill.
Prep the Grill: Preheat an outdoor grill or grill pan to medium high heat (about 400-450°F). Clean and oil the grates.
Grill the Pork: Remove the pork from the marinade and pat it very dry with paper towels. Place it on the preheated grill. Grill for 15-20 minutes, turning every 4-5 minutes to char all sides, until an instant read thermometer inserted into the thickest part reads 145°F.
Rest: Transfer the grilled pork to a clean cutting board, tent loosely with foil, and let it rest for 5-10 minutes.
Slice and Serve: Slice the pork into 1 inch thick medallions. Serve immediately, topped generously with the fresh chimichurri sauce.