I am making French onion soup, and it calls for sherry. What would be a good substitute for the sherry?
Yes, you can. (I usually use wine, in fact). Note that sherries are a fortified wine, so you may want to add a small amount of brandy as well; say 1 Tbl to 1/4 cup per cup of wine.
If you are using a chicken stock, restrict yourself to a white wine; perhaps a rose if you have a particularly dark chicken stock.
For a beef stock, red or white will work well. In addition, if the recipe calls for a sweet sherry, you can also use a port or a Madeira.
In all cases, add in parts and taste between until you're happy with the result.