I don't know, some posters...... - - try this then...:
http://www.thefoody.com/cheese/cheeseandonionp ie.html
Basically the use of a good mustard will really bump up the flavors. No need to add extras but you could chuck in some sweated leek, braised celery, roasted peppers, etc or whatever else takes the fancy...caramalising the onion - reducing by slow cooking for an hour or so over low heat will totally change the texture and flavour, as will adding a smoked paprika, or mixing shallots and onion, or using red and white varieities.....
The quality of the onions by the way will influence the flavour - If you use standard s/market things it will never be as nice as fresh from the field onions or home grown. There are about 700 varities of onion commercially grown, so have a look out for some of the stronger flavour ones rather than the ordinary Spanish.
Try topping the pastry case with grated cheese too - a good aged gouda might be nice...and a sprinkle of paprika through it wouldn't go down badly either.