Step 1 Preheat oven to 190°C. Place capsicums, tomatoes, chilli, onion and garlic in a large ovenproof dish; spray with olive oil and roast, turning and spraying once with olive oil, for 30 minutes. Remove dish from oven and leave to cool for 5 minutes. Squeeze garlic cloves from skin; discard skin.
Step 2 Place half the roast vegies and half the garlic in a food processor; blend for 10–20 seconds, or until mixture is a chunky sauce. Transfer sauce to a medium bowl and add sugar, vinegar and 4 tablespoons water; stir well and set aside.
Step 3 Cook penne in a large saucepan according to packet instructions; drain and return to pan, reserving 1/4–1/2 cup pasta water. Set pan over low heat. Add reserved sauce with remaining roast vegies and garlic; toss well to heat through.
Step 4 Add chicken, mushrooms, basil, rocket and olive oil to saucepan; stir, adding reserved pasta water to thin sauce, if necessary.
Step 5 Divide pasta among 4 plates; scatter with extra basil, parmesan and walnuts to serve.