From my childhood memories they are fried/sauteed mushroom caps.
And for our non-American correspondents, you have caused confusion by calling a Full English Breakfast a Continental Breakfast, which on this side of the pond consists of something pretending to represent an imitation croissant, a cup of weak and tepid coffee, and if one is fortunate a dab of some sort of jelly or jam.
It is reassuring to know that the Americans and the British are still separated by but an ocean and a language.
stay safe.