Cut the onion into a small dice and mince the garlic (or use pre-minced garlic from a jar). Place both in the slow cooker. Also add the crushed tomatoes, tomato paste, brown sugar, balsamic vinegar, bay leaves, basil, oregano, and freshly cracked pepper. Stir well to combine. If your slow cooker tends to lose moisture and dry things out, add 1/2 to 1 cup of water as well.