Place the cranberries in a small bowl and cover with 1 cup warm water. Soak cranberries 1 hour to soften, and then drain and place in a medium bowl. Mix in the walnuts, breadcrumbs and 1 Tbsp. stock or broth.
Preheat the oven to 350 degrees F. Insert a long, thin knife down the centre of the pork to create an end-to-end slit, 1 1/2 inches wide. Stuff the cranberry mixture into the slit. Set pork in a small roasting pan. Brush roast with mustard; season with herbs de Provence and pepper (the mustard will add saltiness).
Roast the pork 50 minutes, or until the centre registers 160 degrees F on a meat thermometer. Set the roast on a plate, tent with foil and rest 10 minutes.
Meanwhile, set the roasting pan on the stovetop over medium-high heat. Add chicken stock/flour mixture. Simmer until a lightly thickened sauce forms; season with salt and pepper. Slice the pork into 8 slices and set 2 on each of 4 plates. Accompany with the sauce.
Note: herbs de Provence is a French-style blend of herbs available in bottled herb and spice aisle.