Mix everything together and let the chimichurri sit for at least 15–20 minutes so the flavors can come together. Adjust vinegar, salt, and pepper to taste.
Mix the dry rub together and coat the flank steak heavily and evenly on all sides, pressing the seasoning into the meat. Let it sit for 20–30 minutes before grilling.
Heat the grill until very hot. Sear the flank steak over high heat for about 4–6 minutes per side, depending on thickness and desired doneness. For the best results, cook to temperature rather than strictly by time—aim for about 130–135°F for medium-rare.
Remove from the grill and let the steak rest for 10–15 minutes. Slice thinly against the grain.
Toss together: Grilled corn, Cherry tomatoes, halved, Cucumber, chopped, Red onion, thinly sliced, Sliced grilled flank steak.
Drizzle generously with the chimichurri vinaigrette and toss everything together. Taste and add more chimichurri, salt, vinegar, or pepper as needed.
Reserve some chimichurri to spoon over the sliced steak right before serving.
