Preheat oven to 180°C/160°C fan-forced. Line a baking tray with baking paper. Using a sharp knife, make a deep cut along 1 side of each chicken breast to form a large pocket (do not cut all the way through). Place 3 basil leaves, 4 semi-dried tomatoes and 4 pieces bocconcini in each pocket. Wrap 2 slices of prosciutto around each breast.
Heat oil in a large frying pan over medium-high heat. Add chicken. Cook for 3 minutes each side or until golden. Transfer to baking tray. Bake for 25 to 30 minutes or until cooked through. Add stock paste and 2/3 cup cold water to pan. Cook, scraping pan, for 5 minutes or until reduced by half.
Place potato and garlic in a large saucepan. Cover with cold water. Bring to the boil over high heat. Boil for 12 to 15 minutes or until potato is tender. Drain. Reduce heat to low. Return potatoes and garlic to pan. Add butter and milk. Mash until smooth.
Serve chicken with mashed potatoes, steamed broccolini and drizzled with pan juices. Season with pepper