What's a good substitute for onion for someone with an onion allergy?
The perfect solution to you - Use the Indian spice called "Asafoetida" or "Hing". It gives a taste which is very much like Onion and Garlic - In fact, stricter practitioners of Hinduism are not allowed to eat onion and garlic (as supposedly they cause mental agitation). Thus, traditional Hindu (Vedic) cooking uses Asafoetida as a subsitute for onion in Indian recipes which almost invariably call for the use of onions.

The spice gives off a slightly funky smell (I like it, but most people describe it as bad), but tastes great. It is easily available in powdered form in Indian grocery stores - and usually comes in small containters such as this:



It has a VERY strong taste, so put a very small amount of it - about a 1/4 of a teaspoon for a dish that's meant or 4 people. Usually in Indian cooking it is quickly fried to get rid of the "raw" taste. I am not sure how well it tastes in salads, you'd have to experiment with it.