Marinate the Steak: Add the dijon, garlic, onion powder, soy sauce, olive oil, and balsamic vinegar together in a large plastic bag or shallow dish. Season both sides of the steak with salt and pepper then add the steak to the marinade let it marinate for 2 hours or overnight in the refrigerator.
Preheat Your Grill: When you’re ready to grill, brush the grill grates with oil and heat the outdoor grill to high heat (450°F). Remove the flank steak from the marinade and shake off excess, but do not discard marinade as you will need it later for making glaze.
Grill the Flank Steak: Add the flank steak to the hot grill and close the lid. Cook for 3-5 minutes on each side for medium-rare steak, or until the internal temperature reaches 135°F, which allows for 5-10 degrees of carryover cooking while it rests.
Prepare Zucchini on Grill: Brush zucchini slices with a little olive oil and sprinkle with salt. Add them to the other side of the grill when you flip the steak, grilling for 2-3 minutes on each side until they are tender and charred in spots.
Resting Time & Make Balsamic Glaze: Remove both steak and zucchini from heat and let them rest on a clean cutting board for about 10 minutes. Meanwhile, add reserved marinade to a medium saucepan and bring it to a rolling boil over medium-high heat until it reaches at least 165°F, ensuring it's safe after marinating.
Whip Feta & Serve: Blend feta cheese and heavy cream until smooth using a blender or food processor. Brush half of your balsamic glaze onto sliced steak before serving atop whipped feta, grilled zucchini, drizzling remaining glaze on top, then enjoy!