Cook the penne in a large saucepan of boiling water for 8–9 minutes, or according to the pack instructions. Drain and refresh under cold running water, then toss with 2 tablespoons of the oil.
Meanwhile, place the Parmesan, tarragon, pine nuts, garlic and lemon rind in a food processor and process for 1 minute. Then, while the machine is running, gradually pour in the remaining olive oil to form the pesto.
Toss the pesto with the pasta, chicken, watercress, tomatoes and lemon juice, and serve.