The vegetables can be served hot, warm, or at room temperature. After about five minutes, faint grill marks should begin to appear on the undersides of the vegetables; if necessary, adjust their position on the grill or adjust the heat level.

1 large red onion , peeled and cut crosswise into four 1/2-inch-thick rounds
3 medium zucchini (about 1 pound), ends trimmed, sliced lengthwise into 3/4-inch-thick planks
1/4 cup plus 2 tablespoons extra virgin olive oil

Kosher salt and ground black pepper

1 small garlic clove , minced or pressed through garlic press (about 1/2 teaspoon)
1 teaspoon finely grated zest from 1 lemon plus 1 tablespoon juice
1/4 teaspoon Dijon mustard
1 tablespoon chopped fresh basil leaves

1. 1. Thread onion rounds, from side to side, onto 2 metal skewers. Brush onion and zucchini evenly with ¼ cup olive oil and season with 2 teaspoons kosher salt (or 1 teaspoon table salt) and pepper to taste.
2. 2. Whisk remaining 2 tablespoons olive oil, garlic, lemon zest and juice, mustard, and ½ teaspoon kosher salt (or ¼ teaspoon table salt) together in small bowl; set aside.
3. 3. Grill vegetables over medium heat, turning once, until tender and caramelized, 10 to 15 minutes. Transfer vegetables to platter; remove skewers from onion and discard any charred outer rings. Rewhisk vinaigrette and pour over vegetables. Sprinkle with basil and serve