Combine the Dijon mustard, minced garlic, onion powder, soy sauce or coconut aminos, olive oil, and balsamic vinegar in a large food-safe bag or shallow dish.

Season both sides of the flank steak with salt and black pepper. Add it to the marinade, coat thoroughly, and refrigerate for at least 2 hours or overnight.

When ready to cook, lightly oil the grill grates and preheat an outdoor grill to high heat, approximately 450°F.

Remove the steak from the marinade, allowing the excess liquid to drip off. Transfer the used marinade to a saucepan and set it aside for later.

Place the steak on the hot grill and close the lid. Grill for approximately 3–5 minutes per side for medium-rare, or until it reaches your preferred safe internal temperature. For medium-rare, remove it at about 135°F and allow for some carryover cooking as it rests.

Brush the zucchini slices with olive oil and season with kosher salt. When you flip the Balsamic Flank Steak, place the zucchini on the grill.

Grill the zucchini for 2–3 minutes per side, or until tender with visible char marks. Gently press the slices against the grates with tongs for more defined grill marks.

Transfer the steak and zucchini to a clean cutting board or platter. Allow the steak to rest for 10 minutes before slicing.

Meanwhile, bring the reserved marinade to a full rolling boil in a saucepan. For food safety, ensure it reaches at least 165°F. Reduce the heat and simmer for 2–3 minutes, or until it thickens enough to coat the back of a spoon. Add another splash of balsamic vinegar if needed.

Place the feta and heavy cream or milk in a food processor or blender. Blend until smooth and creamy.

Brush some of the cooked balsamic glaze over the rested steak. Slice the steak thinly against the grain.

Spread the whipped feta over a serving platter and arrange the grilled flank steak and zucchini on top. Finish with the remaining balsamic glaze and serve.