3 Tablespoons vegetable oil

1 small white onion, thinly sliced

1/2 Cup vinegar

1/2 Teaspoon salt, or to taste

1/2 Cup water

2 Ounces queso anejo, romano, or sardo cheese, finely grated (about 2/3 cup)

1/4 teaspoon dried Mexican Oregano

1 Small Onion, thinly sliced

6 Romaine lettuce leaves

1 Medium Tomato, sliced

A day ahead, heat an ungreased skillet or comal over medium heat and let the chiles heat through, turning them over from time to time, until they have softened and become flexible. Flatten each chile out as much as possible, then make a slit down one side and halfway around the top, to which the stem is attached. Remove the seeds and veins.