Brocho before eating this:
If the onion rings contain a flour coating: Borei Minei Mezonos
If the onion rings do not contain flour: Shehakol Niyeh Bedvoro
Brocho after eating the proper shiur of this: Borei Nefashos [assuming a proper shiur of flour was not eaten]
Source: The Laws of B'rachos (page 375)