Chives are a commonly used herb and can be found in grocery stores or grown in home gardens. In culinary use, the scapes and the unopened, immature flower buds are diced and used as an ingredient for fish, potatoes, soups, and other dishes (Wikipedia, nd).
Chives come from the onion plant, so if you are allergic to onions, you may be allergic to chives as well. Chives can be mixed in many recipes or put on foods as garnish, so always ask specifically for the ingredients if you are unsure. Check with your allergist if you have an allergic reaction (Wikipedia, nd).
Chives have many uses in the culinary world, and can be prepared in various ways, such as slicing and mixing in soups or frying like a chip. Here is some further reading regarding the use of chives.
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Some tasty chive dishes can be found here.