Step 1 Preheat oven to 200°C. Bring a medium saucepan of water to the boil. Add quinoa and cook for 15 minutes, or until tender and opaque; drain and set aside.
Step 2 Meanwhile, spray a medium frying pan with cooking oil; set over medium heat. Add onion, celery and apple; cook, stirring, for 5 minutes, or until celery is soft. Add reserved quinoa and sage; stir to make stuffing.
Step 3 Place turkey breasts, skin-side down, on a clean work surface and butterfly. Open each turkey breast so it lies flat. Spoon stuffing along the centre of each turkey breast; fold sides to enclose stuffing. Tie folded turkey breasts with kitchen string at 2cm intervals to secure.
Step 4 Set a flameproof roasting pan over high heat; add olive oil and heat. Cook turkey for 4–5 minutes per side, or until browned. Pour 1–2 cups water into pan. Transfer pan to oven and roast turkey for 30–35 minutes, or until golden and juices run clear when you insert a skewer into thickest part. Remove turkey from oven, cover with foil to keep warm and set aside to rest for 15 minutes.
Step 5 Meanwhile, drain roasting pan juices into a medium heatproof jug; skim any excess oil. (You’ll need 2 cups pan juices; top up with water if necessary). Set a small saucepan over medium heat and add gravy powder. Whisk pan juices and water into powder; bring to the boil. Cook gravy, stirring continuously, for 2 minutes, or until thick.
Step 6 Cut string from reserved turkey. Slice turkey into medium–thick pieces, divide among 8 plates and serve with gravy.