This is the 293rd most frequent Croatian word.
jednostavno
Simply.
The word 'jednostavno' here is used as an adjective meaning 'simple' or 'easy' to describe the dish.
Ovo je jednostavno jelo za napraviti.
This is a simple dish to make.
In this case, 'jednostavno' works as an adverb meaning 'simply', elaborating on the manner in which to say something.
Samo jednostavno reci što misliš.
Just simply say what you think.
Here, 'jednostavno' is used as an adverb to emphasize the disbelief in the statement.