Rinse the dried black beans thoroughly and soak them overnight in plenty of water, or do a quick soak by boiling for 5 minutes and letting sit for an hour.
Drain the soaked beans and set aside. Dice the onion into small, uniform pieces and mince the garlic cloves finely.
If you have time, sauté the diced onion and minced garlic in a tablespoon of oil in a skillet over medium heat until fragrant and lightly golden, about 5-7 minutes. This deepens their aroma and flavor but can be skipped for a faster prep.
Add the soaked beans to the slow cooker. Pour in the vegetable broth, then stir in the smoked paprika, cumin, and chili powder for a smoky, spicy base.
If you've sautéed the aromatics, add the cooked onion and garlic to the slow cooker now; otherwise, toss in the raw diced onion and garlic directly.
Cover with the lid and set the slow cooker to low. Let cook for 6 to 8 hours, until the beans are tender and the soup has thickened, with some beans breaking down into a velvety base.
Halfway through cooking, check the liquid level. If it’s too thick, add a splash of broth or water. If it’s too thin near the end, uncover and cook on high for an additional 15-20 minutes to thicken.
Once the beans are tender, stir in fresh lime juice and taste. Adjust salt and pepper as needed, then use an immersion blender to puree part of the soup for a creamier texture, if desired.
Serve the soup hot in bowls, garnished with your favorite toppings like cilantro, cheese, or sour cream for added flavor and creaminess.
