Mince the red onion and set it aside.
Add the basil and parsley leaves (no stems) to a blender then process until the herbs are finely chopped. Pause as needed to scrape the herbs from the sides of the blender. (note: a food processor may also be used)
Next add the olive oil, vinegar, lemon juice, salt, red pepper flakes to the blender. Grate in the garlic and continue processing until it still has some chunky texture.
Transfer the chimichurri to a jar or storage container, mix in the minced red onion by hand then taste and adjust salt as necessary. If you prefer a thinner consistency, drizzle in about 1 tablespoon of olive oil.
Serve over grilled chicken, steak, fish or shrimp.