Heat the oil in a wide skillet over medium heat until it shimmers.
Add the chopped onion and cook, stirring now and then, for 5 to 6 minutes until it is soft and translucent and just starting to take on colour at the edges.
Add the garlic and the cumin and cook for about 45 seconds, until fragrant. Do not let the garlic brown.
Add the drained beans and the salt. Stir so every bean is coated in the seasoned fat, then let them heat through for 2 to 3 minutes.
Begin mashing directly in the pan with a potato masher or the back of a sturdy spoon, adding the warm broth a splash at a time as you go. Stop while the mixture still looks slightly looser than you want.
Cook for another 3 to 5 minutes, stirring, until the beans thicken and begin to pull away from the sides of the pan.
Taste for salt. Serve warm, adding another splash of warm broth if they tighten up before they reach the table.