In a food processor, pulse together the herbs, onion, lemon juice, garlic, salt, pepper, and red pepper flakes.
Drizzle the olive oil over the top and process until you have a slightly chunky sauce.
Cover and refrigerate the sauce until almost ready to serve. The sauce will need to be at room temperature for 15-30 minutes prior to serving for the olive oil to re-liquify.
Stir the sauce well and serve.
Notes
Store the chimichurri sauce in an airtight container in the refrigerator for up to 5 days.