1. Drain beans; place in slow cooker. Add onion and crushed garlic. Pour in enough water to cover. Cook, covered, until beans are tender, about 4 hours on high or 6 on low.
2. Drain beans, reserving liquid. Use a potato masher, pastry blender or fork to work bean mixture into a thick, pulpy mass.
3. Warm oil in a large pot over medium-high heat. Add minced garlic and jalapeño. Cook, stirring often, until softened, 3 to 5 minutes. Stir in mashed bean mixture and cook until warmed through, about 5 minutes. Add small amounts of reserved cooking liquid to thin as desired. Mixture should have the consistency of thick mashed potatoes.
4. Remove from heat and stir in lime juice and zest. Season generously with salt and pepper. Top with queso fresco and chopped cilantro, if desired.