1. Put a large saucepan of water on to boil. Combine walnuts, garlic, salt, pepper and cayenne in a blender; pulse until finely chopped. Blend in broth, parsley and lemon juice until smooth and creamy. 2. Heat oil in a medium nonstick skillet over medium-high heat. Add chicken and cook, stirring often, until it is no longer pink in the middle, 3 to 5 minutes. 3. Cook pasta for 5 minutes. Add broccoli and bell pepper, cook, stirring occasionally, until the pasta and vegetables are just tender, about 4 minutes more. Drain, return to the pot. Add the walnut sauce and chicken; toss to coat.