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 ingredients together until just evenly incorporated, being careful not to overwork the meat.
02 - Roll the mixture into 1 1/2-inch meatballs, yielding approximately 20 to 24 uniform pieces. Keep your hands lightly dampened with water to prevent sticking.
03 - For enhanced flavor and texture, heat a thin layer of oil in a large skillet over medium-high heat. Sear the meatballs in batches for 2 to 3 minutes per side until lightly browned on all surfaces. Transfer to the slow cooker.
04 - Place all the meatballs in an even layer at the bottom of the crockpot.
05 - 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. Pour the gravy mixture evenly over the meatballs in the slow cooker.
06 - Cover the crockpot and cook on LOW for 4 to 5 hours or on HIGH for 2 to 3 hours, until the meatballs are cooked through and reach an internal temperature of 160°F.
07 - Stir the heavy cream into the cooking liquid. If the gravy needs thickening, gradually stir in the cornstarch slurry. Set the crockpot to HIGH and cook uncovered for an additional 10 to 15 minutes until the gravy reaches your desired consistency.
08 - 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.