Trim any excess fat from the beef then pound or slice very thin.
Cut the beef into individual slightly larger than palm-size pieces then season with salt and pepper and set aside.
Drizzle one tablespoon of olive oil in a skillet over medium heat then add the onions and spinach. Season with salt and pepper and cook 2-3 minutes or until the onions are tender and spinach wilted.
Add the parsley, breadcrumbs, Parmesan, Mozzarella cheeses and egg to the skillet, blending to form the stuffing.
Divide the stuffing into equal parts for each piece of beef. Lay a slice of prosciutto onto each piece of beef then spread the stuffing over the prosciutto. Roll the beef tightly around the stuffing and secure with kitchen twine or toothpicks.
Add another tablespoon of olive oil to the same skillet over medium-high heat then gently place the rolls in the skillet searing on all sides.
Add the marinara to the bottom of the slow cooker then top with the beef rolls. Deglaze the skillet with the beef stock, scraping any brown bits from the bottom then pour into the slow cooker.
Simmer on high for 3 ½ – 4 hours or ow 6 ½ – 7 hours. Times will vary depending upon thickness of the beef (you want the beef to be fork tender). Slice the rolls pinwheels and serve with pasta.