Cook pasta according to package directions. Drain and set aside.
Meanwhile, in a large skillet, heat oil over medium heat. Add walnuts and garlic. Cook about 2 minutes or until light brown, stirring frequently. Add sweet peppers and red onion. Cook for 5 to 7 minutes or until vegetables are crisp tender, stirring frequently. Add tomatoes; cook and stir for until heated through. Stir in parsley, rosemary, and black pepper. Put pasta in a large shallow bowl. Top with walnut-pepper mixture; toss gently to coat. If desired, sprinkle with Parmesan cheese.