vegetarian french onion soup recipe?
as ghg says take the time to cook the onions properly. they should get the color of caramel. mushroom or veggie stock, or a not-too-dry white wine would all work, i usually add thyme, tarragon or rosemary towards the end of cooking time. and a couple of slices of lemon if the flavor needs brightening. maybe a good idea with mushroom or veggie broth but probably unnecessary if you use wine.
this should help:
and honestly, as long as you take the time to cook the onions properly and substitute good mushroom stock for the beef, you can use pretty much any trusted recipe. plenty to choose from: