Finely shredded cooked chicken or BEEF (optional),
1⁄2 cup Vegetable oil
Using a spoon, place about 1 tbsp. salsa Arbol on top of half the tortillas, spreading it to the edges; repeat with salsa Tomatillo and remaining tortillas. Divide onion among all tortillas, and top with shredded chicken or beef, if you like.
Return skillet to stove, add ½ cup oil, and heat over high heat until it begins to smoke. Working in batches, add tortillas, salsa sides up, and cook, basting tops with oil, until crisp at the edges, about 1 minute. Using a metal spatula, transfer tortillas to a serving platter; serve immediately.