Step 1 Preheat oven to 200°C. Line a baking tray with baking paper. Place potatoes on tray and bake for 30 minutes, or until golden and tender, adding walnuts to tray for the last 5 minutes.
Step 2 Meanwhile, set a medium saucepan over medium heat; add stock and bring to a simmer. Add chicken; cook for 7 minutes, or until almost cooked through. Remove poached chicken from heat; leave to cool for 15 minutes.
Step 3 Bring a small saucepan of water to the boil. Add beans; cook for 1–2 minutes, or until just tender. Refresh beans in cold water; drain.
Step 4 Put mayonnaise, basil and lemon juice in a food processor; blend until smooth. Cut reserved chicken into thin slices.
Step 5 Toss potato, walnuts, poached chicken, beans and tomato in a large bowl. Serve on top of salad greens alongside a dollop of basil mayonnaise.