Prepare Spinach-Basil Pesto; set aside. In a medium skillet heat oil over medium heat. Add mushrooms; cook for 3 to 5 minutes or until tender, stirring occasionally. Add tomatoes, onion, garlic, basil, and the 1/2 teaspoon salt; cook and stir for 1 minute more. Set aside.
Sprinkle chicken lightly with additional salt and pepper. In a medium nonstick grill pan cook chicken over medium heat for 12 to 15 minutes or until no longer pink (170 degrees F), turning once.
Meanwhile, cook spaghetti according to package directions; drain. Return spaghetti to hot pan. If desired, add spinach and arugula mix; toss just until wilted. Add pesto; toss gently to coat. Gently stir in mushroom mixture.
To serve, slice chicken. Divide spaghetti mixture among dinner plates. Top with chicken.
In a food processor or blender combine basil, spinach, cheese, oil, garlic, and salt. Cover and process or blend until nearly smooth, stopping to scrape down sides as necessary. Season to taste with pepper. Store, covered, in the refrigerator for up to 3 days or freeze for up to 1 month.