Preparation Heat lard in a large heavy pot over medium-high heat. Add onion and cook, stirring occasionally, until edges are charred, 8–10 minutes. Meanwhile, cook jalapeño in a small dry skillet over high heat, turning occasionally, until blackened, 8–10 minutes; cut a slit in jalapeño. Add jalapeño, epazote sprig, if desired, and beans to pot; add water to cover by 2". Simmer, partially covered, until beans are creamy, 2–3 hours (depending on freshness of beans). Season with salt and cook 15 minutes longer; discard epazote.
Preparation Heat lard in a large heavy pot over medium-high heat. Add onion and cook, stirring occasionally, until edges are charred, 8–10 minutes. Meanwhile, cook jalapeño in a small dry skillet over high heat, turning occasionally, until blackened, 8–10 minutes; cut a slit in jalapeño. Add jalapeño, epazote sprig, if desired, and beans to pot; add water to cover by 2". Simmer, partially covered, until beans are creamy, 2–3 hours (depending on freshness of beans). Season with salt and cook 15 minutes longer; discard epazote.