01 - Whisk together balsamic vinegar, olive oil, minced garlic, chopped rosemary, Dijon mustard, kosher salt, and black pepper in a shallow dish or zip-top bag. Add the flank steak, turning to coat thoroughly.
02 - Refrigerate the steak for at least 1 hour, or up to 8 hours for optimal flavor development.
03 - Heat your grill or grill pan to medium-high temperature.
04 - Brush zucchini planks with olive oil and season evenly with kosher salt and black pepper.
05 - Remove steak from marinade, allowing excess to drip off. Grill for 4–5 minutes per side for medium-rare, or adjust timing to reach desired doneness. Transfer to a cutting board and tent loosely with foil for 10 minutes.
06 - While steak rests, grill zucchini for 2–3 minutes per side until tender and lightly charred.
07 - Combine crumbled feta cheese, Greek yogurt, olive oil, lemon juice, and black pepper in a food processor or blender. Process until completely smooth and creamy.
08 - Slice the rested steak thinly against the grain. 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.