Make the chimichurri: In a blender or food processor, combine the garlic, jalapeños, vinegar, parsley, oregano, and olive oil. Blend until well combined but still slightly chunky. Season generously with salt and pepper.
Season the pork tenderloin all over with salt and pepper. Place in a dish and coat with half of the chimichurri. Let marinate for 30 minutes at room temperature. Reserve the remaining chimichurri for serving.
Preheat your grill or a grill pan to medium-high heat (375°F/190°C).
Grill the pork for 15-20 minutes total, turning every 5 minutes to cook evenly on all sides, until the internal temperature reaches 145°F/63°C.
Remove the pork from the grill and let it rest, tented with foil to keep warm, for 5 minutes.
Slice the pork evenly into medallions and serve with the reserved chimichurri sauce.
