01 - Whisk together balsamic vinegar, olive oil, garlic, rosemary, Dijon mustard, salt, and pepper in a shallow dish or zip-top bag. Add flank steak, turning to coat thoroughly.
02 - Refrigerate the steak for at least 1 hour, up to 8 hours for optimal flavor penetration.
03 - Heat grill or grill pan to medium-high temperature.
04 - Brush zucchini planks with olive oil and season evenly with salt and pepper.
05 - Remove steak from marinade, allowing excess to drip off. Grill for 4–5 minutes per side for medium-rare doneness, adjusting time for preferred doneness.
06 - Transfer steak to a cutting board, tent loosely with foil, and let rest for 10 minutes to allow juices to redistribute.
07 - Grill seasoned zucchini for 2–3 minutes per side until tender and marked with char lines.
08 - Combine feta cheese, Greek yogurt, olive oil, lemon juice, and black pepper in a food processor. Blend until completely smooth and creamy.
09 - Slice steak thinly against the grain. Spread whipped feta on a serving platter, arrange grilled zucchini and steak slices on top. Garnish with fresh herbs and balsamic glaze.