In a food processor or blender, add all chimichurri ingredients. Process until well combined and desired consistency reached. Reserve ½ of the sauce and refrigerate for serving.
Place Flank Steak in a glass baking dish. Pour remaining marinade over the top and rub marinade over the entire surface. Flip steak and repeat. Marinate steaks for at least 30-60 minutes, flipping once.
Remove steak from marinade and lightly pat with paper towels to remove excess liquid. Grill steak, covered, for 7 to 11 minutes for medium rare (145°F) doneness, turning once. During the last 5-6 minutes, add the tomatoes and corn to the grill. Cook, flipping once, until slightly tender and golden.
Let steak rest for 5-10 minutes. Carve steak lengthwise in half, then crosswise, against the grain, into thin slices. Top tortillas evenly with steak slices and vegtables. Drizzle with additional Chimichurri and sprinkle with leftover cilantro.