Start by finely chopping the parsley and mincing the garlic and shallot if using. Aim for small, even pieces so the sauce has a balanced texture.
Add the chopped parsley, garlic, oregano, red wine vinegar, diced chile (or red pepper flakes), salt, and black pepper to a medium bowl. Mix everything together until evenly combined.
Pour in the olive oil and stir thoroughly until the mixture becomes glossy and well blended. The consistency should be loose enough to spoon easily.
Let the chimichurri sit at room temperature for about 15 minutes. This resting time allows the flavors to meld and develop fully.
Give the sauce a final stir before serving. Use it as a topping, dip, or marinade depending on your preference.
If using a food processor, pulse the ingredients briefly until chopped but not pureed, then stir in the olive oil by hand for the best texture.