Start by patting the flank steak dry with paper towels, which helps it sear properly. Rub it with olive oil, salt, and black pepper to enhance its natural flavor. Let it sit at room temperature for about 10 minutes before cooking.
Heat a grill pan or grill over medium-high heat until hot. Place the steak on the grill and cook for about 4–5 minutes per side, depending on your preferred doneness. Once done, transfer it to a plate and let it rest for at least 5 minutes.
In a small bowl, whisk together olive oil, balsamic vinegar, soy sauce, Dijon mustard, and minced garlic. Mix until the dressing is smooth and well combined. Taste and adjust seasoning if needed.
In a large bowl, combine mixed greens, cherry tomatoes, sliced red onion, and avocado. Toss gently to mix everything evenly without bruising the greens. This creates a fresh and colorful base.
After resting, slice the steak thinly against the grain for maximum tenderness. Cutting it this way ensures each bite is juicy and easy to chew. Keep slices uniform for better presentation.
Add the sliced steak on top of the prepared salad base. Sprinkle crumbled feta cheese over everything for a tangy finish. Drizzle the dressing evenly across the salad.
Toss lightly just before serving or leave layered for a more elegant look. Serve immediately while the steak is still slightly warm. Enjoy the perfect balance of flavors and textures.