Trim edges of the turkey breast of any excess fat. With the skin side down, make a horizontal cut 3/4 of the way through the turkey then open it up like a book. Season well with salt and pepper, and evenly smear a thin layer of the mashed potatoes. Top potatoes with a thin layer of the stuffing. Leave a border around the edge so the breast can be rolled up nicely without overstuffing. Roll the breast up and secure in four separate places with butcher twine. Trim the roast on the ends so it has a nice even shape and to ensure even cooking. Season the exterior with salt and pepper.
Preheat an oven to 400ºF.
Set a large, ovenproof roaster pan over high heat. Add a drizzle of oil and when smoking hot, add the turkey to the pan and brown all over – about 2-3 minutes. Transfer the whole pan to the oven and roast for 25-35 minutes. Once internal temperature reaches 165ºF, remove turkey from oven and let rest.
Pour out the juices from the turkey pan and reserve. Set the roasting pan over medium-high heat and add the mushrooms. Brown well for 5-7 minutes. Add butter and flour then stir to make a roux. Cook until lightly browned – about 8-10 minutes. Add reserved roasting juices back to the pan, along with chicken stock and stir well to combine. Simmer and reduce until it is nice and thick then finish by swirling in butter. Slice turkey into thick pinwheels and serve topped with porcini mushroom gravy.