Step 1: In a large bowl, gently combine the ground beef, 1/4 of the sliced onion, panko breadcrumbs, egg, minced garlic, 1 tbsp Worcestershire sauce, 1/2 tsp dried thyme, salt, and pepper. Do not overmix; mix just until combined.
Step 2: Shape the meat mixture into an oval loaf and set aside.
Step 3: Lightly grease your slow cooker insert. Place half of the remaining thinly sliced onions on the bottom of the slow cooker.
Step 4: Carefully place the shaped meatloaf on top of the onions. Arrange the rest of the sliced onions around and over the meatloaf.
Step 5: Pour the beef broth and remaining 1 tbsp Worcestershire sauce around the meatloaf in the slow cooker. Sprinkle with the remaining 1/2 tsp dried thyme.
Step 6: Cover and cook on LOW for 4-6 hours or on HIGH for 2.5-3.5 hours, or until the internal temperature of the meatloaf reaches 160°F (71°C).
Step 7: During the last 20-30 minutes of cooking, open the slow cooker and sprinkle the shredded Swiss cheese evenly over the top of the meatloaf. Re-cover and continue cooking until the cheese is melted and bubbly.
Step 8: Carefully remove the meatloaf from the slow cooker. Let it rest for 5-10 minutes on a cutting board before slicing and serving with the savory gravy from the crockpot.
Step 9: Garnish with fresh parsley, if desired, and serve hot.