Step 1With the food processor running, drop the garlic through the food tube to chop. Add the parsley, oregano and fresh mint and pulse several times to mince. Add the olive oil, vinegar, lemon juice and salt and pulse a few times to achieve a coarse puree. Taste and add more lemon juice or salt as needed. (The recipe can be prepared to this point 2 hours in advance and held at room temperature.)
Step 1Pat the flank steak dry on both sides with paper towel. Season each side with a generous one-fourth teaspoon salt and several grinds of pepper.
Step 2Just before cooking, brush each side with about 1 tablespoon chimichurri sauce. Grill over a hot fire, cooking about 1 minute on each side, then turning the steak 90 degrees and cooking the same side an additional minute before flipping the steak over to give each side nice grill marks. Repeat with the other side, cooking the steak a total of 4 to 6 minutes. Remove from heat.
Step 3Rest the steak 5 minutes before slicing. Cut across the grain and on a deep bias, in wide, thin slices. Serve, passing remaining chimichurri sauce on the side.