Place the flank steak inside a large resealable plastic bag. Add the cranberry sauce and balsamic vinegar, followed by the sliced white onion, minced garlic, and fresh rosemary sprigs.
Seal the bag securely and refrigerate the steak for at least 3 hours. For deeper flavor penetration, allow it to marinate for as long as 8 hours.
While the beef marinates, place the softened cream cheese, dried cranberries, minced shallot, chopped walnuts, chopped rosemary, and crumbled blue cheese in a food processor or blender.
Pulse the mixture for approximately 30 seconds, just long enough to combine the ingredients and break them down slightly. Set the prepared filling aside until needed.
Remove the steak from the marinade and season it with the sea salt and black pepper. Allow the meat to come to room temperature.
Butterfly the flank steak by carefully slicing it through the middle widthwise without cutting completely through the opposite side. Open the steak and gently pound it with a meat mallet until the thickness is relatively even.
Distribute the prepared cheese and cranberry filling evenly across the opened steak.
Roll the filled steak firmly into a compact log, then secure it at intervals with kitchen twine.
Enclose the rolled steak tightly in plastic wrap and refrigerate it for 10 to 15 minutes so it can firm up and retain its shape.
Heat a frying pan over medium-high heat.
Scatter the freshly cracked peppercorns over a plate. Unwrap the chilled steak roll and turn it in the cracked pepper until the exterior is coated.
Place the rolled steak in the heated pan and sear for approximately 2 minutes per side, developing a well-browned, flavorful crust around the exterior.
Preheat the oven to 350°F (175°C).
Transfer the seared steak roll to an oven-safe baking dish and place it in the preheated oven.
Bake for approximately 20 minutes for medium-rare doneness. Adjust the baking duration slightly according to the desired level of doneness.
Remove the stuffed flank steak from the oven and allow it to rest undisturbed for 10 minutes so the juices can redistribute throughout the meat.
After resting, cut the rolled steak into individual rounds and serve.