1. Preheat grill to medium-high (about 450°F). To prepare chimmichurri, whisk together the sherry vinegar, zest, juice, and 1/4 cup of the oil in a small bowl. Stir in the parsley, cilantro, mint, shallots, and 1/4 teaspoon each of the salt and pepper.
2. To prepare steak, combine the ancho chile powder, red chile flakes, and remaining 3/4 teaspoon salt
and 1/2 teaspoon pepper in a small bowl. Rub the steak with the remaining 2 tablespoons oil; sprinkle both sides of the steak with the spice mixture.
3. Grill, uncovered, to desired degree of doneness, 2 to 3 minutes per side. Let stand 10 minutes; cut diagonally across the grain into thin slices. Serve with the chimichurri.
