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 and turn to coat thoroughly. Marinate in refrigerator for at least 1 hour, up to 8 hours for optimal flavor development.
02 - Heat grill or grill pan to medium-high temperature, approximately 400-450°F.
03 - Brush zucchini planks with olive oil and season evenly with salt and pepper.
04 - Remove steak from marinade and let excess drip off. Grill for 4-5 minutes per side for medium-rare doneness, or adjust timing to reach desired internal temperature. Transfer to a cutting board, tent loosely with foil, and rest for 10 minutes.
05 - While steak rests, grill zucchini planks for 2-3 minutes per side until tender with visible char marks.
06 - Combine feta cheese, Greek yogurt, olive oil, lemon juice, and black pepper in a food processor or blender. Process until completely smooth and creamy, scraping down sides as needed.
07 - Slice rested steak thinly against the grain at a 45-degree angle. Spread whipped feta across a serving platter, arrange grilled zucchini and steak slices on top. Garnish with fresh herbs and drizzle with balsamic glaze if desired.