Step 1

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.

Step 2

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.

Step 3

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.