Ok, so I need shallots and realized I didn't have any!!! AHHHH! What can I use for a substitute?!
It really depends on the recipe you are making. How are you using the onion? What's the recipe?
Shallots are amazing - not onions and not garlic. Any onion can be used as substitute and recommend you make the recipe again using shallots and taste if there is a difference.
You'll probably be OK with onion, but it will be more pungent, so go cautiously. Most of the shallots available are actually hybrids with a type of red onion; true shallots are pretty small. If you have red onion, that would be better.