Strain beans, reserving the broth.
Heat lard and/or oil in a large cast-iron or heavy-bottomed skillet until very hot, you’ll want about ¼ cup fat for every 4 cups of beans.
If using lard, it will start to turn golden (which is good!).
Add onions until they begin to brown around the edges.
Then, add the beans and some of their broth, about 1 cup for every 4 cups of beans.
Simmer for a few minutes and start mashing with a wire masher until the beans are velvety.
Continue mashing until very smooth.
Cook the beans for longer to thicken, or add more broth to thin.
