Preheat the broiler. Place the onion, garlic, and chiles on a baking sheet and broil, turning, for 5 to 7 minutes, until blackened and blisetered on all sides. Remove from the broiler and transfer to a cutting board. Chop coarsely.
Combine the cumin and coriander in a mortar or coffee grinder and grind to a coarse powder.
Rinse and sort through the beans. Place in the slow cooker and add the water and beer. Add the onion mixture and the spice mixture and stir well. Cover and cook on low for 8 hours, or until the beans are very tender.
Heat a large saute pan over medium-high heat and add the oil. Spoon the beans into the pan, a bit at a time, and smash them with a potato masher. Gradually add all the beans, then any remaining liquid, and cook until some of the liquid evaporates and the beans thicken. The finished beans should have some texture to them. Stir in salt to taste and serve warm. (If you prefer a smoother texture, puree in a food processor before transferring them to the saute pan.)