In a large mixing bowl, combine the bread cubes, Parmesan cheese, garlic, onion, parsley, rosemary, and thyme.

Season with salt and pepper, and add the olive oil, mixing until the ingredients are well combined.

Add the beaten egg to the mixture to help bind the stuffing together. The stuffing should be moist but not too wet; if needed, add more breadcrumbs or a splash of chicken broth.

Set aside while you prepare the prime rib.

Preheat your oven to 450°F.

Using a sharp knife, create a deep pocket in the center of the prime rib without cutting through the meat. This pocket will hold the stuffing.

Generously season the entire roast with salt and pepper.

Stuff the pocket with the stuffing mixture, pressing it firmly inside. Be careful not to overfill.

In a small bowl, mix the olive oil, garlic, rosemary, and thyme. Spread the Dijon mustard along the prime rib. Then, rub the spice mixture on the outside of the prime rib.

Place the stuffed prime rib on a roasting rack inside a roasting pan.

Roast the prime rib for 20 minutes until a golden-brown crust begins to form. Then reduce the oven temperature to 325°F. Continue roasting until the internal temperature reaches 125°F for medium-rare, which should take about 1 ½ to 2 hours, depending on the size of the roast.

Once done, remove the prime rib from the oven and tent it loosely with foil. Let it rest for at least 15 to 20 minutes before slicing.