Add the mint, parsley, garlic, olive oil, red wine vinegar, red pepper flakes and salt to a blender or food processor.
Pulse until the ingredients are well chopped, but not puréed. Be careful not to blend for too long as the blades can heat up and cause the bright green mint to turn dark. Use a spatula to scrape down the sides to process evenly.
Transfer the chimichurri sauce to a small bowl or jar and stir in the diced red onion.
Taste for seasoning and add more salt, red wine vinegar and pepper flakes as desired for your taste.
Cover plastic wrap directly on top of the chimichurri and store in the refrigerator until you are ready to use it.
This chimichurri recipe yields about 1 cup of sauce and will be more than enough for multiple racks of lamb.Store in the refrigerator for up to 2 days.