Image credit to: Boston Organics
Mix 3 Tbsp salt with 4 qts water and pour brine over dried beans. Allow to soak for 8-24 hrs. If making the soup sooner, bring the brine mixture and beans to a boil for 5 minutes, remove from heat and let sit for 1 hour before cooking.
Drain beans, add broth and cook with salt, bay leaf and garlic cloves until tender (1-2 hours in a heavy pot or 20-25 minutes in a pressure cooker). You want the beans to be tender and on the verge of falling apart. Broth should have a dark, silky texture from the beans.
Meanwhile, pour oil into another pan over medium heat. Add onions. Saute for 5 minutes.
Add cumin. Cook, stirring, 1-2 minutes until cumin is slightly darkened and fragrant. Add poblano peppers and tomatoes. Cook 10 minutes until tomatoes begin to break down.
Once beans are tender, pour into pot with onion, pepper and tomato mixture. Add sweet potatoes, orange peel and juice. Season with cayenne and chili powder.
Cook 20-30 minutes until sweet potatoes are tender and squared edges have just begun to break down into soup. Add broth or water for desired consistency. Add vinegar for brightness. Taste and adjust seasoning.
Garnish with avocado slices, cilantro, sour cream and/or green chile slices and serve.