Heat oven to 375°F. Combine sour cream, mayonnaise, milk, garlic and horseradish in small bowl. Sprinkle with black pepper. Cover and refrigerate until time of serving.
Place potatoes and onion in a shallow roasting pan or on a baking sheet; toss with oil until well coated. Arrange in a single layer; season with salt and pepper.
Place pork loin filet on top of potato mixture, bacon side up. Roast about 1 hour and 15 minutes until internal temperature reaches 150°F. Remove pork from pan and place on cutting board. Let stand 10 minutes before slicing very thin.
Divide pork and serve with potato mixture and the aioli.