Step 1 Spray a 19cm-square cake tin with oil and set aside. Combine stock and milk with 1½ cups water in a medium saucepan. Bring to the boil. Slowly add polenta, whisking continuously. Cook, stirring constantly, for 5 minutes, until thickened. Pour mixture into prepared tin. Cool slightly, then refrigerate for about 1 hour, or until firm.
Step 2 Steam green beans until just tender. Drain, then refresh under cold water. Combine beans, eschalots, olives and tomatoes in a bowl. Toss gently to combine. Set aside.
Step 3 Combine piquillo peppers, spinach and mozzarella in a small bowl. Cut a horizontal slit in the thickest part of each chicken breast to create a pocket. Fill pockets with spinach mixture.
Step 4 Heat barbecue to medium. Chargrill chicken for 10–15 minutes, or until cooked through. Meanwhile, trim edges of polenta. Cut into 8 triangles and spray with oil. Chargrill for about 5 minutes each side, until crisp. Serve chicken with polenta triangles and green bean salad.