⅓ cup walnuts
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
Prepare grill for medium-high heat. Toast walnuts in a dry small skillet over medium heat, tossing often, until fragrant, about 5 minutes. Chop very coarsely. Toss warm walnuts with garlic, lemon juice, and 3 tablespoons oil in a large bowl; season with salt and pepper.
Brush leeks and zucchini with remaining 2 tablespoons oil; season with salt and pepper. Grill vegetables, turning often, until tender and charred in spots, 5–8 minutes for leeks, 8–10 minutes for zucchini.
Transfer vegetables to a cutting board. Trim roots from leeks and cut leeks and zucchini into bite-size pieces. Add vegetables and parsley to bowl with walnuts and toss to combine; season vegetables with salt, pepper, and more lemon juice, if desired.