Yeah but in Flipverse what "go together" is out the window.
Yes, in the same game there's also onion flavoured (Onion Overdrive, though that's up for debate) ice cream, and actual pieces of onion inside of cookies (Onion Zest), so any sort of normal logic as to this stuff is pretty much out of the picture.
What about Sarge's Revenge sauce? I think it probably tastes like onions and Crushida Pepper, as Sarge's full name is Sergeant Crushida Pepper, he's known for throwing pepper bombs and he's an onion.