Chimichurri Steak Sandwich

In the bowl of a food processor add cilantro, 1/2 cup basil, red onion, garlic, 1 teaspoon salt, red pepper flakes, and vinegar. Pulse a few times and scrape down sides of the bowl. While motor is running, drizzle in olive oil and process until almost smooth, but leave some texture. Set aside. Preheat grill on medium-high heat. Drizzle steak with olive oil and season with salt and pepper. Place on grill and cook until lightly charred, about 3 minutes on each side. Transfer to a clean plate and loosely tent with foil; let the meat rest. Meanwhile, toast bread on grill until slightly charred. Slice steak thinly on diagonal against the grain. Spread chimichurri on toasted bread. Layer sliced steak with tomatoes and a few basil leaves. Top with toasted bread to close the sandwich.