Although most places that have at least a hot plate to prepare at least basic food on the premises would be happy to rustle up a plain omelette (tortilla francesa), even if it's not on the menu. It's happened to me many times when travelling with vegetarian friends or relatives. For vegans - or just because it's bloody delicious - toasted bread rubbed with tomato (and garlic if you fancy) and drizzled with with olive oil is a great option. It's more a Catalan thing, but a lot of places elsewhere will have little portions of smushed tomato, which is inauthentic but does the job.