This traditional tapa is not anything like a Mexican tortilla. A Spanish tortilla is a large, thick, omelet-like cake made with potatoes and eggs and is served at room temperature. Romesco sauce is an all-purpose Spanish sauce that is served with a wide variety of dishes. From the Tarragona region, this Catalan sauce combines two of the most popular horticultural imports from the New World---chiles and tomatoes. The sauce gets its name from the romesco chiles that are used but are not readily available outside of Spain. The combination of ancho and New Mexican chiles approximates the taste.