That doesn't really sound like an argument that it's frivolous, it sounds like an argument about why the company shouldn't need to pay much. What if the onions weren't obvious? I don't know if they put their onions in a sauce, in the bun or something else.

It's entirely plausible that lifting the bun would have revealed the onions, even most likely. I wouldn't, however, say that the concept of difficult to spot onions is so unreasonable as to say the case is frivolous.