Preheat the oven to 425 degrees F. Place the pork in a roasting pan; season with salt and pepper. Roast 20 minutes. Reduce heat 325 degrees F; roast 45-60 minutes more, or until pork reaches an internal temperature of 160 degrees F. When cooked, place the roast on a plate, tent with foil and let rest 10 minutes before thinly slicing. Drain the excess fat from the pan and set on the stovetop over medium-high. Whisk the stock and flour together in a bowl. Pour into the pan and bring to a simmer; simmer 5 minutes. Divide the bread slices among plates. Mound slices of the pork on top. Drizzle with the gravy and serve.