Step 1 Preheat oven to 200°C. Layer potato slices and half of the onion slices in a 1-cup-capacity ramekin. (Potato will shrink when baked.) Add chicken stock, cover ramekin with foil and bake for 30 minutes; remove foil. Bake for 15 minutes, or until potato is tender.
Step 2 Meanwhile, place a medium frying pan over medium–high heat. Add broccolini with 1/4 cup water and cook for 3 minutes, or until broccolini is tender yet firm and water has evaporated. Add zucchini and remaining onion; cook, stirring, for 3 minutes, or until zucchini is tender. Add garlic, chilli (if using) and half of the olive oil; cook, stirring, for 1 minute, or until mixture is fragrant and green vegetables are coated. Transfer greens to a serving plate.
Step 3 Heat remaining olive oil in same frying pan set over medium–high heat. Sprinkle chicken fillets with seasoning; cook for 2–3 minutes per side, or until chicken is golden and cooked through.
Step 4 Serve chicken and gratin with greens and a lemon wedge.