Place pumpernickel bread on a large baking pan and crumble into coarse crumbs. Bake, stirring once, 12 to 15 minutes or until lightly toasted. Set aside. Increase oven temperature to 450°F.
Melt the butter in a large skillet over medium heat. Add the onion and cook, stirring often, 5 minutes or until softened. Stir in the garlic and cook 1 minute. Remove skillet from heat and stir in the toasted pumpernickel crumbs, red cabbage, raisins, 1/4 tsp of the salt, and 1/4 tsp of the pepper.
Make a cut lengthwise down the centre of the pork, cutting to, but not through, the bottom. Open the butterflied portions, laying pork flat. Starting from the centre of each half, slice lengthwise down the centre of the pork, cutting to, but not through, the bottom. Unfold pork so it lies flat. Place plastic wrap over meat and flatten to 3/4-inch thickness using a meat mallet or rolling pin.
Sprinkle top of pork with 1/4 tsp of the salt and 1/4 tsp of the pepper and spread with 3 tbsp of the Bavarian mustard. Spread the stuffing mixture evenly over pork and roll up, jelly roll fashion, starting with the long side. Secure with string and place seam side down in a shallow flameproof roasting pan.
Sprinkle outside of pork with the remaining 1/4 tsp of the salt and 1/4 tsp of the pepper and brush with 2 tbsp of the remaining Bavarian mustard. Bake for 20 minutes. Reduce oven temperature to 350°F. Bake 25 to 30 minutes, or until a thermometer inserted diagonally 2 inches into pork (do not touch stuffing) registers 150°F. Transfer pork to a serving platter, cover loosely with foil, and let stand 15 minutes or until thermometer registers 160°F.