Heat the oil in a large skillet over medium high heat. Add the chili powder, cumin, and onion. Cook until the onion is tender, about 5 minutes. Add the beans plus ¾ cup water. Smash the beans with the back of a spoon or a potato masher. Cook until the beans are heated through. If they are too thick, add a little more water. If too thin, cook a little longer. Season to taste with salt and pepper. Serve topped with the crumbled cheese, if using.