You will spot many (former) camino peregrinos here as it is a pretty easy-to-order, straightforward…read morerestaurant/cafe. Premise: they have 100 "montaditos" which are like tiny sandwiches, and you write down which ones you want on a slip of paper, give it to the cashier, and there is your food!
They price very fairly, with Mondays even having all montaditos for 50 cents. The tapas-esque Spanish cuisine really shines in this modern twist. By the way, this is a chain and therefore is also more "reliable" if you are unsure about the restaurant quality around the rest of Santiago. Or, if you were like me still on American time, this place is open even during "siesta."