Place the roast into the slow cooker. In a bowl, whisk together tomato juice, garlic, dried onion, au jus mix, Italian dressing mix, black pepper, and optional cayenne. Pour that mixture over the meat, cover, and cook on LOW until it becomes fall-apart tender.
Once done, remove the meat and let it rest a few minutes. Use two forks to shred the roast, then return it to the slow cooker to soak up more of the sauce.
Heat olive oil in a skillet and stir in the Arborio rice, coating each grain. Add one cup of warmed broth at a time, stirring constantly until absorbed before adding the next. Continue until all broth is used. Finish by stirring in the Parmesan cheese, onion powder, and pepper.
Spoon the risotto into bowls or plates, then top generously with shredded roast and ladle extra sauce over. Garnish if desired and enjoy immediately while hot.