01 - Whisk together balsamic vinegar, olive oil, garlic, rosemary, Dijon mustard, salt, and pepper in a shallow dish. Add flank steak, turning to coat thoroughly. Marinate in refrigerator for at least 1 hour, up to 8 hours for optimal flavor penetration.
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, allowing excess to drip off. Grill for 4-5 minutes per side for medium-rare doneness, adjusting time for preferred internal temperature. Transfer to cutting board and tent with foil; rest for 10 minutes.
05 - While steak rests, grill zucchini for 2-3 minutes per side until tender and nicely charred.
06 - Combine feta, Greek yogurt, olive oil, lemon juice, and black pepper in food processor. Blend until completely smooth and creamy, approximately 1-2 minutes.
07 - Slice rested steak thinly against the grain. Spread whipped feta across serving platter. Arrange grilled zucchini and steak slices atop feta. Garnish with fresh herbs and drizzle with balsamic glaze if desired.