Rinse the mixed beans under cold water and remove any debris. Place them in the slow cooker.
Add the diced onion, carrots, celery, and minced garlic to the slow cooker.
Pour in the canned diced tomatoes, including the liquid.
Add the vegetable broth or water and stir to combine all ingredients.
Sprinkle in the ground cumin, smoked paprika, black pepper, salt, and add the bay leaf. Stir well.
If using, add the chopped kale or spinach to the mixture.
Cover the slow cooker and set it on low heat for 8 hours, or high heat for 4 hours.
Once done, remove the bay leaf and taste the soup for seasoning adjustments.
Serve hot.