Preheat the oven to 180°C/Gas 4.
In a bowl mix the mascarpone, crushed garlic, lemon zest and herbs and season with sea salt and black pepper. Loosen the skin away from the turkey breast and stuff the mascarpone mix underneath. Smooth the skin down then brush the turkey all over with the rape seed oil and season with sea salt and black pepper. Put the lemon halves in the cavity. Put the onion slices in the bottom of the roasting tin and sit the turkey on top. Cover with foil and cook for 30 minutes per kilo, basting the turkey occasionally.
Approximately 30 minutes before the full cooking time, remove the foil, baste the turkey with the juices then turn up the oven to 200°C/Gas 7 and cook for a further 30-35 minutes, until the turkey is golden brown. Remove the turkey from the oven and rest for 20 minutes before carving.
Meanwhile, for the gravy; drain off most of the fat from the roasting tin, place over a low heat and sprinkle in the flour, whisk well and cook for 1-2 minutes, then add the wine and stock and bring to a boil. Strain into a clean pan and simmer for 10 - 15 minutes whilst the turkey rests and adjust seasoning and serve with the turkey.