2 (14.5 ounce) cans white beans, rinsed and drained
1/2 onion, unchopped
3 cloves whole garlic cloves
2 teaspoons kosher salt
1 teaspoon ground black pepper
2 sprigs fresh thyme
1/2 pound Portuguese andouille sausage, sliced
2 bunches kale, torn into small pieces
1 stalk celery, diced
2 carrots, chopped, or more to taste
1/4 teaspoon garlic salt, or to taste
Combine the beef stock, chicken stock, beans, onion half, garlic cloves, salt, black pepper, and thyme in a slow cooker; cook on Low for 8 hours to overnight. Remove and discard the onion and garlic.
Switch slow cooker to 'Keep Warm' setting. Add the andouille sausage to the stock mixture; continue cooking until you can smell the sausage, about 1 hour. Stir the kale, celery, carrots, and garlic salt into the soup; cook until the carrots are tender, about 1 hour more.