01 - In a large mixing bowl, combine the ground beef, breadcrumbs, milk, egg, chopped onion, minced garlic, Parmesan cheese, parsley, salt, and pepper. Gently fold the mixture until just incorporated, being careful not to overwork the meat.
02 - Roll the mixture into 1 1/2-inch meatballs, yielding approximately 20 to 24 uniform portions. Arrange them on a tray or plate as you work.
03 - For enhanced flavor and a firmer texture, heat a thin layer of oil in a large skillet over medium-high heat. Brown the meatballs in batches, about 2 to 3 minutes per side, until a golden crust forms. Transfer to the slow cooker. This step is optional but highly recommended.
04 - In a medium bowl, whisk together the beef broth, cream of mushroom soup, Worcestershire sauce, soy sauce, dried thyme, onion powder, and garlic powder until smooth and well blended.
05 - Place the meatballs in the slow cooker. Pour the gravy mixture evenly over the meatballs, ensuring they are well coated. Cover and cook on LOW for 4 to 5 hours or on HIGH for 2 to 3 hours, until the meatballs are cooked through and tender.
06 - During the last 15 minutes, stir in the heavy cream. If the gravy needs thickening, whisk the cornstarch and cold water together, then stir the slurry into the slow cooker. Cook on HIGH for an additional 10 to 15 minutes until the gravy reaches your desired consistency.
07 - Taste the gravy and adjust seasoning with salt and pepper as needed. Serve the meatballs and gravy hot over mashed potatoes, egg noodles, or steamed rice.