"He thought about eating onion."
Translation:On pomyślał o jedzeniu cebuli.
It's Genitive, as needed by "jedzeniu" (eating of what?). If he just thought about onion, it would be Locative indeed.
do you jeść coś or czegoś? I can see this being "he thought about eating onions" because I think that is the plural form of onion, right? Cebuli? But if you jeść coś and it is only one onion... Shouldn't it be "On Pomyślał o jedzeniu cebulę."