In a large resealable plastic bag or shallow dish, combine Dijon mustard, minced garlic, onion powder, soy sauce, olive oil, and balsamic vinegar to create the marinade. Add the flank steak, ensuring it is well-coated. Seal the bag or cover the dish, and marinate in the refrigerator for at least 2 hours or overnight for deeper flavor penetration.

About 30 minutes before cooking, remove the marinated flank steak from the refrigerator to bring it to room temperature. Preheat your grill to high heat, aiming for around 450°F.

Place the marinated flank steak on the hot grill. Cook for 3-5 minutes on each side for medium-rare, using an instant-read thermometer to check for a temperature of 135°F.

While the steak is grilling, prepare the zucchini by brushing it with olive oil and seasoning with salt. Grill for 2-3 minutes per side, until it is tender and has a lovely char.

Once the steak reaches the perfect doneness, remove it from the grill and let it rest on a cutting board for about 10 minutes, then slice against the grain.

In a small saucepan, pour the remaining marinade from the steak and bring it to a boil over medium heat. Cook for 2-3 minutes until it thickens into a glaze.

To serve, place a dollop of whipped feta on your plate, layer the sliced flank steak on top, add the grilled zucchini, and drizzle with balsamic glaze.