mushy peas vs side dish

mushy peas and side dish both are nouns.

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

As nouns, side dish is a hypernym of mushy peas; that is, side dish 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 dish: a dish that is served with, but is subordinate to, a main course

Other hypernyms of mushy peas include entremets, side order.

mushy peas (noun)	side dish (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