1. 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.
2. 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.
3. 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.
4. Divide pork in half and serve with potato mixture and half the aioli. Reserve remaining half of pork and aioli for Long Leaf Pork Lettuce Wraps recipe.