In large bowl, soak bread in milk until absorbed, about 5 minutes. Mix in sausage until well combined.
Meanwhile, arrange pork on cutting board with narrow end closest to you. Holding knife parallel to cutting board and starting at right side, cut in half horizontally almost but not all the way through; open like a book.
Step-by-step: How to slice pork tenderloin
Starting at centre, slice left side in half horizontally almost but not all the way through; open like a book. Turn and repeat on other side. Place between waxed paper or plastic wrap; pound to even 1/2-inch (1 cm) thickness. Spread mustard over top.
Arrange sausage mixture in 2-inch (5 cm) wide strip lengthwise along centre of pork. Starting at long side, roll up; tie at 2-inch (5 cm) intervals with butcher's twine. Sprinkle with salt and pepper.
In large skillet, heat oil over medium-high heat; cook pork, turning, until browned all over, about 5 minutes. Place, seam side down, on rimmed baking sheet. Bake in 375 F (190 C) oven until juices run clear when pork is pierced and instant-read thermometer inserted in centre reads 160 F (71 C), about 25 minutes. Transfer to cutting board; tent with foil. Let rest for about 5 minutes before slicing.
Creamy Mustard Sauce
: Meanwhile, in same skillet, heat oil over medium heat; cook onion and garlic, stirring often, until softened and golden, about 4 minutes. Add wine; cook until reduced by half, about 3 minutes. Stir in apple cider, cream, mustard, salt and pepper; bring to boil. Reduce heat and simmer until thick enough to coat back of spoon, about 5 minutes. Serve with pork.