Preheat oven to 375°F Butter a 13x9-inch glass baking dish.
Place pork in center of baking dish, tucking thin end underneath.
Arrange potatoes (white and sweet) around pork. Sprinkle with salt and pepper. Bake for 20 minutes.
Meanwhile, in a large skillet, melt butter over medium-high heat. Add mushrooms, garlic, onions, paprika and 1/4 teaspoons each salt and pepper; sauté for 8 minutes or until mushrooms are brown.
Stir in broth and mustard.
Whisk flour into milk; stir into pan and bring to a boil, stirring. Boil, stirring for 1 minute.
Pour sauce over pork and potatoes and roast for 10-15 minutes or until a meat thermometer inserted in thickest part of pork registers160°F
Transfer pork to a cutting board; let rest for 5 mintues, Return potatoes and sauce to oven for 5 minutes; stir well. Slice pork crosswise; serve with potatoes and sauce. Sprinkle with parsley.
FOR THE ADVENTUROUS: Use mixed gourmet mushrooms, 1/2 teaspoons smoked paprika along with 1 teaspoons regular paprika and add 1 tablespoons chopped fresh thyme with the mustard.