It depends how big your bin is and indoors/outdoors.
With a bigger bin, if you put the onion/garlic in a corner, the worms have room to avoid it and wait till it is "worm ready". Once they're mushy and gooey and loose the "bite" they worms willl start on them. I don't remember where I read it but it's the gasses that will harm the worms. And these gasses are also what we, the wormfarmers don't like to have indoors.

I've never had any problems feeding onion scraps to my indoor rubbermaid bin. As with anything moderation is the key and I don't think I'd add a whole lot in one feeding.

Here is a link that might be useful: BorealWormer

I read somewhere that if onions are introduced to worms that haven't previously been eating them, they probably won't like them. But the next generation hatched in the same bin with onions will be used to them from birth, and will have no problem eating them. I found this to be true. The first batch of worms I ordered from a company wouldn't touch onions. But a couple of months later, onions disappear just like everything else.

I threw in a bulb of garlic for the first time several weeks ago because the cloves were shriveled. As of today the garlic skins are gone, but the cloves are still there, untouched. I'm counting on the next generation of hatchlings to take care of them for me.

Today I also found several onion skins and ends that had worms nestled in them so thickly that I couldn't separate them out and had to throw them back into the bin worms and all.