Something I have noticed, with my own cats.
Cats do NOT like onions.

Occasionally they will come looking to see if they can get a tidbit while I
am cooking.
I'll tell them that kittys don't like onions, and offer to let them sniff.
They always scrinch up their noses and squint their eyes and back up a step
or two.
Maybe run some onion through your blender and mix it with water then
sprinkle it out and about.