Combine the garlic, shallots, lemon and vinegar in a bowl and let sit for 10 minutes. This step is optional, but will add a slightly better flavor to your sauce.
Combine all remaining ingredients (except olive oil) in a food processor and blend until a sauce forms. Don't blend until totally smooth, you want some chunks in the sauce.
Stir in the olive oil at the end (not using the food processor) and let it sit for about 15 minutes to give the flavors a chance to meld together. Serve at room temperature.
Notes
Wash the herbs thoroughly to remove any dirt.
Always use fresh lemon juice and a good quality olive oil for the best flavor.
Don't use dried herbs - fresh herbs should always be used when making chimichurri.
I don't recommend substituting the red wine vinegar. However, if you HAVE to, substitute it with white wine vinegar.
For a spicier sauce, add more red chilies or chili flakes.
Don't blend the sauce until totally smooth, you want it to be slightly chunky.
You can experiment with different herbs like oregano, thyme, etc.