In a large skillet on the stove-top, cook the ground beef and onions together, crumbling the ground beef with a wooden spoon. Drain off any excess cooking fat (if there is any) and add to a 4 quart or larger slow cooker.
Add the remaining ingredients (ketchup, vegetable juice, Parmesan cheese, garlic powder, oregano and Italian seasoning) to the slow cooker and stir to combine.
Cover and cook on LOW for 1 to 2 hours or until everything is hot and bubbling.
Serve hot Stromboli mixture on sliced Italian or French bread (*see note if on a gluten-free diet) with shredded or sliced mozzarella cheese on top.
*As is, the Stromboli meat mixture is gluten free, to make gluten free sandwiches simply use your favorite gluten free bread such as Schar Gluten-Free Baguette.