1. Bring a large pot of salted water to a boil.
2. In a blender, combine walnuts, garlic, 1/2 teaspoon salt, 1/4 teaspoon pepper and cayenne and pulse until finely chopped. Add broth, parsley, cilantro and lemon juice and blend until smooth and creamy. Set aside.
3. Season chicken breasts with salt and pepper. In a small nonstick skillet, heat oil over medium heat. Add chicken and cook until well browned and no longer pink inside, 4 to 5 minutes per side. Slice into thin, diagonal slices and place in a large serving bowl. Toss with reserved walnut sauce.
4. Boil farfalle for 12 minutes. Add broccoli and bell peppers and cook until pasta is al dente and vegetables are tender, about 4 minutes more. Drain well, transfer to bowl with chicken and toss to coat. Taste and adjust seasonings. Serve hot.