Preheat a grill or a grill pan over medium-high heat. I like to use a dual-contact electric grill.
Trim the zucchini edges and slice it crosswise into ½-inch-thick rounds.
In a small bowl, whisk the olive oil, balsamic vinegar, kosher salt, black pepper, and garlic powder.
Brush the zucchini rounds with the mixture on both sides. Set aside any remaining marinade.
Grill the zucchini slices until just tender, 3-4 minutes on each side. In my dual contact grill, I grill them for 3 minutes on the first side, then for 2 more minutes on the second.
Brush the zucchini slices with the remaining marinade and serve.
Video
Notes
You want your zucchini nice and firm, not limp and mushy. The best way to make it crispy on the grill is to slice it thickly (½ inch thick), get the grill hot (but not too hot), and avoid overcooking the zucchini.
You can use a teaspoon of fresh minced garlic instead of garlic powder. However, garlic powder works better in this recipe: It is more uniformly distributed and doesn't burn on the grill as quickly as minced garlic.
The leftovers can be stored in the fridge in a sealed container for 2-3 days. It's best to use them cold as antipasti. If you prefer to reheat them, do so gently in the microwave at 50% power.