Shallots taste very similar to a mild onion, and one of the best substitutes are green onions. Remember to use the green of the green onions, all the way to the tip. There is alot of flavor in the greens and adds great color to your dish.
1 Additional Answer
Shallots, by taste, are basically mild onions. So conceivably, one could use onions in any recipe that calls for shallots. Scallions are also very similar to shallots.
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