Uncountable name of vegetables can be used as:
1) Name of vegetable species.
2) A mass of vegetables which can be measured in kilograms, for example. (Like water or milk)

If you want to describe one solid object you can use different word or phrase and often there is an official (compound) name for such a countable object and a colloquial one.

Here is a table:

uncountable - official countable - colloquial countable

картофель - клубень картофеля - картофелина, картошка
капуста - кочан капусты - капустина
лук - луковица - луковица
чеснок - головка чеснока - чесночина
свекла - ...
морковь - ... - морковка
редис - ... - редиска
горох - стручок гороха/горошина
фасоль - стручок фасоли/боб - фасолина