Preheat the oven to 425 degrees F.
Place the roasts in a shallow roasting pan. Brush with the mustard; sprinkle with salt, pepper and rosemary. Roast 20 minutes, and then reduce temperature to 325 degrees F. Roast 35 to 40 minutes more, or until the centre of the pork registers 160 degrees F on a meat thermometer. Set the roasts on a plate, tent with foil and rest 10 minutes.
Meanwhile, set the roasting pan on the stovetop over medium-high heat. Add the apples and cook, stirring, for two minutes. Pour in the brandy, simmer and reduce by half. Add stock/flour mixture, bring to a simmer, and simmer a few minutes until a thickened sauce forms. Season the sauce with salt and pepper. Thinly slice the pork and serve with the sauce.