1/3 cup vegetable oil
2 pounds potatoes, scrubbed and diced
1 medium onion, diced
1 poblano chile, seeded and sliced thin
2 medium tomatoes, peeled and coarsely chopped
Salt, to taste

Heat oil in a large heavy skillet. Add potatoes, stir to coat with oil, then pat into a flat layer. Cook over medium heat until they begin to sizzle and turn golden around the edges, about 10 minutes. Stir in the onion and chile and pat into a flat layer again. Cook over medium heat until the potatoes are browned on the bottom, about 5 minutes. With a spatula, turn the potatoes over to brown the other side.

If using tomatoes, stir them in and pat into a layer again. Sprinkle with salt to taste, raise the heat to medium-high, and cook until the potatoes are nicely browned on the bottom again, about 5 minutes.

If using cheese, sprinkle over the top and cook 2 minutes more, or until the cheese softens. Serve immediately.