Prepare the marinade by mixing Dijon mustard, minced garlic, onion powder, soy sauce, olive oil, and balsamic vinegar. Season the flank steak with salt and pepper, then marinate for at least 2 hours or overnight.
Preheat the grill to high heat. Remove the flank steak from the marinade, shake off excess, and grill for about 3-5 minutes on each side for medium-rare doneness.
Brush zucchini slices with olive oil and sprinkle with kosher salt. Add them to the grill when flipping the steak for the second time, grilling for 2-3 minutes on each side.
Let the steak rest for about 10 minutes after grilling to allow juices to redistribute.
Pour the reserved marinade into a saucepan and bring to a boil, then reduce to a simmer and let thicken for 2-3 minutes.
Blend feta cheese with heavy cream or milk until smooth to create whipped feta.
Slice the rested flank steak against the grain, plate with whipped feta and grilled zucchini, and drizzle with balsamic glaze.