Preheat the oven to 425°F.
While the oven is heating, place the breadcrumbs, minced garlic, chopped onion, melted butter, and parsley in a bowl. Mix until the ingredients are thoroughly combined to form the stuffing.
Position the tenderloin on a work surface and cut it lengthwise using a pinwheel-style technique, gradually unrolling the meat as you slice.
Begin near the top of the tenderloin while holding the knife blade at an angle. Make an initial cut approximately 1 inch deep, then gently roll the meat away as you continue cutting.
Proceed slowly with cuts approximately 1–2 inches at a time, continually opening and flattening the tenderloin against the work surface. Continue until the roast is sufficiently opened to receive the filling and can later be rolled back into a pinwheel.
Distribute the prepared breadcrumb stuffing in an even layer across the opened tenderloin.
Carefully roll the beef back into a compact roast, enclosing the filling as you work. Secure the rolled tenderloin firmly with butcher’s twine so that the stuffing remains in place.
Season the exterior evenly with the kosher salt and freshly ground black pepper.
Set the tied tenderloin on a wire rack positioned inside a roasting pan. Transfer it to the preheated oven and roast for 50–70 minutes, adjusting the cooking time according to the desired degree of doneness.
For a final internal temperature of 145°F, cook the beef to medium-rare. For medium doneness, aim for a finished internal temperature of 160°F.
Remove the tenderloin from the oven when its internal temperature is approximately 10°F below the intended final temperature, since residual heat will continue cooking the meat as it rests.
Allow the roasted tenderloin to rest undisturbed for 10 minutes. This period permits the juices to redistribute while the internal temperature continues to rise.
After resting, remove the butcher’s twine as needed, cut the stuffed tenderloin into portions, and serve.