1 garlic clove, finely grated
2 tablespoons fresh lemon juice
5 tablespoons olive oil, divided
Kosher salt and freshly ground black pepper
2 large leeks, white and pale-green parts only, halved lengthwise with some root attached
2 large zucchini (about 1 lb.), halved lengthwise
½ cup (lightly packed) fresh flat-leaf parsley leaves with tender stems