noun
pea
PEE/piː/
A small round edible seed or the plant that produces it.
Round green peas fill the open pod.

Etymology
Pease could name one seed or the crop collectively. Speakers reinterpreted it as a plural and created pea as a new singular.
In context
So split pea soup is half done. Looking pretty good. So at this point, halfway through the cooking, I'm going to add another half a cup of split peas.