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 until just incorporated — avoid overworking the mixture to keep the meatballs tender.
02 - Roll the mixture into 1 1/2-inch meatballs, yielding approximately 20 to 24 uniform pieces. Place them on a tray or plate as you work.
03 - For enhanced texture and richer flavor, heat a thin layer of oil in a large skillet over medium-high heat. Sear the meatballs in batches, 2 to 3 minutes per side, until lightly browned. Transfer to the slow cooker.
04 - Arrange the shaped (or browned) meatballs in an even layer at the bottom of the crockpot.
05 - In a separate 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.
06 - 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 have absorbed the flavors of the gravy.
07 - Stir in the heavy cream. If the gravy is thinner than desired, add the cornstarch slurry and stir gently. Cook on HIGH for an additional 10 to 15 minutes until the gravy reaches a rich, coating consistency.
08 - Taste and adjust seasoning as needed. Serve hot over creamy mashed potatoes, buttered noodles, or steamed rice. Pair with green beans or a crisp side salad.