Chimichurri is kind of like pesto with different herbs. I've had it in restaurants and it's made with parsley (I think), but since we like cilantro so much, we thought this one was great. Use half the sauce for a marinade for the steak, and reserve about a tablespoon per serving to spoon on top of the meat after it's cooked.
a large bunch of cilantro, about 1 1/2 cups packed
3 cloves of garlic
1/2 cup olive oil
2 teaspoons white wine vinegar
1 teaspoon dried oregano
1/2 teaspoon cumin
1/2 teaspoon red pepper flake, optional
1/2 jalapeno pepper, membranes and seeds removed
Combine cilantro, garlic, vinegar, oregano, cumin, jalapeno, and red pepper flakes in a food processor. Pulse until fine.
Turn food processor on and stream in olive oil. Season with salt and pepper to taste.
For the flank steak: marinade meat for an hour or longer, up to overnight, in the chimichurri (reserve about a 1/4 of a cup for serving). Grill steak on the highest heat for about 4-6 minutes per side, depending on the thickness of the meat. Flank steak should only be cooked to medium, but medium rare is better. Slice the steak against the grain after letting it rest for 5 minutes. Spoon the reserved chimichurri over the top of the steak.