Place the onion, rinsed beans, jalapeno, garlic, salt, pepper, and cumin into a slow cooker. Pour in the water and stir to combine. Cook on High for 8 hours, adding more water as needed. Note: if more than 1 cup of water has evaporated during cooking, then the temperature is too high.
Once the beans have cooked, strain them, and reserve the liquid. Mash the beans with a potato masher, adding the reserved water as needed to attain desired consistency.