The appetizer platter was scheduled to have five ingredients. The menu explained the potato-nutmeg meatballs and the deep-fried cheese balls. We already knew our cheeses, edam and gouda. What neither of us knew was what a roll mop was. The platter was due to come with more than one. We guessed literally: a roll to mop up the grease wth?
When the platter arrived, the waitress mumbled something about the chef giving us extra meatballs because they'd run out? run low? Something to do with fish? We weren't sure what. From context, we guessed they'd run low on cheese balls, and had given us extra meatballs in compensation. The platter comprised a dressed leaf salad, surrounded by lovely greasy cheese and meat balls, substantial slices of cheese, and a pile of thin slices of a smooth, rich, cured meat. By elimination, therefore, the sliced meat had to be the roll mops. (Although we still weren't sure quite what the waitress had said.)
Our deduction made perfect sense under the circumstances, but now - having come home and looked it up - I know that that's not what roll mops are at all! They're pickled herring rolled up around a piece of cucumber or onion, a kind of herring canapé. Summer Isles Foods - last heard from here as purveyors of whisky-cured salmon, sell ready-made ones.