Heat the oil in a medium pan over medium-high heat until it shimmers. Add the onion and cook, stirring often, until it's soft and browned at the edges, about 5 minutes. Add the beans, garlic, and chile powder.
Let the beans come to a brisk simmer, then lower the heat to maintain a gentle simmer. Cook, stirring and mashing often until the beans resemble a very coarse puree and have thickened, 15 to 20 minutes. When you tip the pan, the beans should creep forward like lava. The beans will thicken a little more once they cool. Add salt to taste. Use warm or at room temperature.
