Marinate the flank steak for at least 4 hours or up to 12 hours.
For the chimichurri, combine all ingredients, except lemon, in a food processor and pulse until pureed (the consistency – roughly pureed or smooth, depends on your personal preference).
Grill flank steak on high heat for about 2 to 3 minutes on each side for medium-rare, depending on the thickness. Add lemons to the grill and place flesh side down until partially charred.
After grilling, let the flank steak rest before slicing, then slice into long strips, be sure to cut across the grain. Squeeze the lemon juice out of the charred lemon into the chimichurri, and drizzle chimichurri over the flank steak.