I am going to be cooking for someone who, because of Crohn's disease, is unable to eat pretty much anything from the allium family (ie. onions, garlic, shallots, chives, scallions and leeks; this includes garlic and onion powder), as well as spicy food (so that excludes chilis and piquant spices). I have seen Substitute for onions and garlic but because the suggestions there are pretty much other alliums, it is not very helpful for my purposes.
Because onions tends to be in a lot of things (if nothing else, as half of a mirepoix, which tends to appear in many recipes, and is also the base of stocks, which makes things complicated -- I am not 100% sure if the two degrees of separation from onion to stock to whatever the stock is used for is enough to make it "safe", and unfortunately I won't have time to make an onion-less stock), I am not really sure what I can use instead.
I realize that it might be hard to substitute onions and garlic with other similar tasting ingredients without using other alliums, so instead, I am looking for suggestions to replace onions and garlic with things that will still let me cook flavourful food (that doesn't necessarily need to taste the same as it would with alliums, though).
For example, I was thinking about roasting a chicken and making a couscous dressing, but the chicken would sit on mirepoix (can I replace the onions in it with something else?) which would be used as a base for the sauce as well; and my couscous dressing recipe uses shallots (as well as sage, thyme, oregano and pine nuts). General suggestions would be useful as well, though.
Any suggestions would be greatly appreciated!