What can I substitute for onions? I love onions; however, someone that I cook for does not.  Absolutely hates them and can tell whenever they are in the dish. 

That said, so many recipes call for onion to be added. What can I use instead that would have a similar but less intense/noticeable effect?
Leeks can provide some of the same flavors as onions, but the flavors are lower intensity than onions.