In Argentina, where grilled beef (“asado”) is an art form, chimichurri is an essential condiment—meats, like this Grilled Tri-Tip, are never served without it. Here, it’s a bright-green, pesto-like sauce containing herbs, garlic, and tangy vinegar (note that, while fresh oregano adds an earthy punch, dried is an acceptable substitute). If you can, make the chimichurri a day before serving to give the flavors time to meld.
1Place parsley, garlic, oregano, vinegar, red pepper flakes, salt, and pepper (to taste) in the bowl of a food processor fitted with a blade attachment. Process until finely chopped, stopping and scraping down the sides of the bowl with a rubber spatula as needed, about 1 minute total.
2With the motor running, add oil in a steady stream. Scrape down the sides of the bowl and pulse a few times to combine. Transfer sauce to an airtight container and refrigerate at least 2 hours or up to 1 day to allow the flavors to meld. Before serving, stir and season as needed. The chimichurri will keep in the refrigerator for up to 1 week.