Take the turkey breast out of the fridge and allow it to come to room temperature for an hour.
Heat the oven to 190C/170C fan/gas 5. Put a rack inside an oven tray with the onions and carrots underneath, or arrange the veg inside an ovenproof frying pan.
Weigh the turkey breast and calculate 40 mins per kilo, plus an additional 20 mins.
Rub the butter over the skin and season well. Put the turkey breast on the rack or rest it on top of the veg in the pan.
Pour in enough water to cover the veg and cover the whole tin or pan with a tent of foil.
Roast for the allotted time, taking the foil off 20 mins before the end to brown the skin. Test with the point of a knife and see if the point feels hot as soon as you pull it out (be careful) and the juices run clear. If you have a thermometer then it should read 65-70C. If the joint appears to be underdone, then put back in the oven for another 10 mins.
Leave the turkey to rest for 20 mins somewhere warm, it will keep cooking so the final internal temperature will rise to 70C, or just above that. Don’t skip this step otherwise the juices will run out as you carve.
Use the strained veg and liquid in the bottom of the tin and the juices from carving to add to this gravy, if you like, and serve with the turkey and veg.