Combine ground beef, bread crumbs, beaten egg, minced garlic, dried thyme, salt, and pepper in a mixing bowl. Mix gently until just combined, then shape into small meatballs about 1 inch in diameter.
Place the meatballs on a plate or tray and set aside. In your slow cooker, pour in the broth and add diced carrots, celery, and chopped onions.
Gently add the meatballs into the slow cooker, making sure they’re submerged in the broth and vegetables. Cover with the lid and set to low heat.
Cook uncovered for about 5-6 hours, or until the meatballs are cooked through and the flavors meld. The broth will become rich and fragrant, with the meatballs maintaining a tender, juicy texture.
Check the seasoning and adjust with salt and pepper as needed. If desired, sprinkle fresh herbs at the end for added flavor.
Serve the soup hot, ladling the broth with meatballs and vegetables into bowls. Enjoy the warm, hearty flavors with crusty bread or a simple side.