In the slow cooker, combine all the ingredients except for the optional greens (spinach or kale), lemon juice, parsley, salt, and pepper.
Stir everything together in the slow cooker until well mixed.
Cover the slow cooker with the lid and set it to cook on HIGH for 4-5 hours or on LOW for 8-10 hours.
About 5 minutes before the soup is done, stir in the optional greens (spinach or kale) and let them wilt.
Season the soup with salt and pepper to taste.
Squeeze the juice of 1 lemon into the soup and give it a final stir.
Ladle the soup into bowls and garnish each serving with a sprinkle of fresh chopped parsley.
Serve hot and enjoy! This soup pairs wonderfully with homemade vegan cornbread, vegan naan, or artisan bread.