Spanish omelette vs tortilla

Spanish omelette

noun
  • A molded omelette made with the addition of fried potatoes and often onions. 

tortilla

noun
  • Spanish omelette; an omelette containing potatoes and onions. 

  • A flat round bread made out of cornmeal or flour. 

How often have the words Spanish omelette and tortilla occurred in a corpus of books? (source: Google Ngram Viewer )