Make the marinara sauce: Combine the crushed tomatoes, onions, garlic, chili powder, balsamic vinegar, olive oil, dried oregano, salt, and pepper in the bowl of the slow cooker. Stir to combine.
Prepare the meatballs: Combine the ground beef, breadcrumbs, egg, dijon mustard, oregano, salt and pepper in a medium bowl. Use your fingers to work the meatball mixture together. Work only until all the ingredients are evenly combined.
To shape the meatballs, pinch off a few tablespoons of meatball mixture and shape into balls with your hands. Place them in the tomato sauce. Scoop a little sauce over the top of the meatballs so that they're coated and submerged.
Cook for 4 to 6 hours on low. The meatballs are done when they're no longer pink in the middle. Serve over spaghetti with freshly grated parmesan cheese sprinkled on top.