In a large food-safe bag or shallow nonreactive dish, combine the Dijon mustard, minced garlic, onion powder, soy sauce, 3 tablespoons olive oil, and balsamic vinegar.
Season both sides of the flank steak with salt and black pepper. Add it to the marinade and turn to coat thoroughly. Cover 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 (232°C).
Remove the steak from the marinade, allowing the excess marinade to drip off. If you plan to turn the used marinade into a glaze, transfer it immediately to a saucepan for thorough boiling; never use it uncooked.
Place the steak on the hot grill and close the lid. Grill for approximately 3–5 minutes per side, depending on thickness and preferred doneness. Use an instant-read thermometer rather than relying only on cooking time.
Brush the zucchini slices lightly with 2–3 tablespoons olive oil and season with kosher salt. When you flip the steak, place the zucchini on the grill.
Grill the zucchini for approximately 2–3 minutes per side until tender with visible char marks.
Transfer the cooked steak and zucchini to clean plates or cutting boards. Keep cooked food separate from any utensils or surfaces that touched the raw steak. Let the steak rest for about 10 minutes.
Meanwhile, bring the reserved used marinade to a full rolling boil in a saucepan. Continue boiling thoroughly before reducing the heat and simmering for several minutes until the mixture reduces into a glossy glaze that coats the back of a spoon. For an easier alternative, discard the used marinade and prepare a separate fresh batch for the glaze.
Place the feta and heavy cream or milk in a food processor or blender. Blend until creamy and smooth, scraping down the sides as necessary.
Brush some of the balsamic glaze over the rested steak. Slice the Balsamic Flank Steak with Zucchini and Whipped Feta thinly against the grain.
Spread the whipped feta over the serving plate. Arrange the sliced steak and grilled zucchini on top, then finish with the remaining balsamic glaze.