01 - In a large mixing bowl, combine the ground beef, breadcrumbs, milk, egg, chopped onion, minced garlic, Parmesan cheese, parsley, salt, and pepper. Mix gently with your hands until just incorporated, being careful not to overwork the meat.
02 - Roll the mixture into uniform 1 1/2-inch meatballs, yielding approximately 20 to 24 pieces. Arrange them on a tray or plate as you work.
03 - For enhanced flavor and texture, heat a thin layer of oil in a large skillet over medium-high heat. Brown the meatballs in batches, turning every 2 to 3 minutes until lightly crusted on all sides. Transfer to the slow cooker as each batch finishes.
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 all meatballs in the slow cooker. Pour the gravy mixture evenly over the top, ensuring the meatballs are mostly submerged. Cover and cook on LOW for 4 to 5 hours or on HIGH for 2 to 3 hours, until the meatballs are cooked through to an internal temperature of 160°F.
06 - Stir the heavy cream into the slow cooker. If the gravy needs thickening, drizzle in the cornstarch slurry while stirring gently. 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 white rice.