1. Combine ground beef, breadcrumb, egg, minced garlic, dried thyme, and a pinch of salt and pepper in a large mixing bowl. Mix gently until evenly incorporated, being careful not to overmix. Form mixture into small meatballs about 1 inch in diameter and set aside.

2. Place the prepared meatballs in the slow cooker evenly, making sure they aren’t crowded to ensure even cooking.

3. Add diced tomatoes, chicken broth, sliced carrots, chopped celery, and diced onion to the slow cooker. Stir gently to combine, ensuring the vegetables are submerged in the broth.

4. Cover the slow cooker with the lid and set to low heat. Let the soup simmer for about 4 hours until the meatballs are cooked through and the vegetables are tender.

5. Once cooking time is up, open the lid and add chopped kale if desired. Stir gently and let it wilt for about 5 minutes, allowing the greens to soften and absorb flavors.

6. Use a ladle to serve the hearty soup into bowls, ensuring each portion contains tender meatballs and a mix of vegetables. Adjust salt and pepper seasoning to taste before serving.