Preheat oven to 350°F. Spread 1/2 cup roughly chopped walnuts on a rimmed cookie sheet; bake until golden, 8 to 12 minutes, stirring once. Let cool on a wire rack. Place walnuts, 1 clove garlic, 1 1/2 cups fresh mint leaves, 1/2 cup fresh basil leaves, 2 Tbsp. crumbled feta (1 oz.), 1/2 tsp. lemon zest, 2 Tbsp. fresh lemon juice, and salt and pepper to taste in a food processor; pulse to mix. With machine running, add 1/2 cup olive oil and mix until mixture is smooth. Serve with lamb. Serves 10.