Preheat the oven to 425°F (220°C). Prepare your ingredients while the oven heats up.

In a large mixing bowl, combine breadcrumbs, minced garlic, diced onions, melted butter, and chopped parsley. Mix until evenly combined.

Slice the beef tenderloin lengthwise into a pinwheel shape without cutting all the way through.

Spread the stuffing mixture evenly over the beef tenderloin, leaving a small border around the edges.

Roll the tenderloin back into its original shape and tie tightly with kitchen twine.

Season the outside of the tenderloin generously with salt and pepper.

Place the stuffed tenderloin in a roasting pan and roast for 50-70 minutes, or until the internal temperature reaches about 125°F.

Cover loosely with foil and let it rest for 15-20 minutes before slicing.