The first in what I anticipate will be a recurring series.
Me, in a sandwich shop: Me gustaría un bocadillo de pollo con mostaza.
The woman working at the counter: ¿Un qué?
Me: Un bocadillo de pollo. Con mostaza.
Woman: Ah, quieres un sándwich de pollo.
And further proof that The Great American Sandwich is most definitely A Thing here right now, this was the cover of the menu where I ate lunch yesterday: