mushy peas vs side order

mushy peas and side order both are nouns.

Word	Noun	Adjective	Verb	Adverb
mushy peas	Yes	No	No	No
side order	Yes	No	No	No

As nouns, side order is a hypernym of mushy peas; that is, side order is a word with a broader meaning than mushy peas:

mushy peas: marrowfat peas that have been soaked overnight and then boiled; served with fish and chips
side order: a dish that is served with, but is subordinate to, a main course

Other hypernyms of mushy peas include entremets, side dish.

mushy peas (noun)	side order (noun)
marrowfat peas that have been soaked overnight and then boiled; served with fish and chips	a dish that is served with, but is subordinate to, a main course