1Cook and drain spaghetti as directed on package.
2Meanwhile, in blender or food processor, place parsley, 1/2 cup of the walnuts, the garlic, lemon peel, salt, oil and yogurt. Cover; blend about 3 minutes, stopping occasionally to scrape down sides with rubber spatula, until smooth.
3Toss pesto with cooked spaghetti. Divide among serving bowls. Top evenly with tomatoes, cheese and remaining 1/4 cup walnuts.