Step 1 Preheat oven to 200ºC. Place spinach in a large heatproof bowl. Just cover with boiling water and stand for 1 minute, then rinse under cold running water. Drain well, squeeze excess water from spinach and finely chop.
Step 2 Place ricotta, spinach and sage into a small bowl and stir until well combined. Season with ground black pepper. Cut a deep pocket in the side of each chicken breast, being careful not to cut all the way through. Stuff each pocket with 1/4 ricotta mixture. Secure with a toothpick.
Step 3 Spray a large frying pan with oil and place over high heat. Add chicken and cook for 2 minutes each side, until golden brown. Transfer chicken to a large baking tray and bake for 12–15 minutes, or until cooked through.
Step 4 Meanwhile, boil sweet potato in a large saucepan for 15 minutes, or until tender. Drain well, return to pan, add milk and mash until smooth. Serve chicken (removing toothpick before serving) with mash and steamed vegetables.