Pick through and rinse beans. Optional: soak overnight.
In a pot, combine beans, cumin, salt, oregano or epazote, half an onion, and garlic. Cover with water.
Bring to a boil, then simmer until tender, about 1½ hours. Add hot water as needed.
Strain beans, reserving cooking liquid.
Heat lard or oil. Sauté minced onion until browned. Add jalapeño and cook 1 minute.
Add beans to skillet. Mash while adding bean liquid in batches to reach desired consistency.
Season with salt, cumin, and oregano to taste. Serve warm.