Recipe: Onion Tortilla

1 lb. of Spanish onions, 1-1/2 oz. of butter or oil, 3 eggs. Melt the butter in a frying-pan, slice the onions, and fry them for 10 or 15 minutes, beat the eggs, add them to the onions, season with pepper and salt, and fry the whole a light brown on both sides.

The Allinson Vegetarian Cookery Book, by Thomas R. Allinson (Year 1915)