In a small bowl, combine parsley, red onion, garlic, oregano, and red pepper flakes.
Add the apple cider vinegar and lemon juice, if using.
Slowly drizzle in the olive oil while stirring.
Season with salt to taste.
Stir until fully combined.
Let the chimichurri sit for at least 10–15 minutes before serving to allow the flavors to meld.
Serve over grilled meats, seafood, vegetables, sandwiches, or grain bowls.