A well-made pot of chicken soup is one of the most universal of comfort foods, a steaming, savory taste of home. The version cherished in the Western world has a rich chicken flavor, enhanced and broadened by herbs and, usually, an aromatic mixture of onions, celery and carrots. The vegetables should remain distinctly secondary, leaving the chicken's own flavor front and center. Celery is especially assertive, and it can give the both a bitter flavor if it's overused. Resourceful cooks can correct this error in several ways.
Addition by Subtraction
Take action as soon as your taste buds tell you the celery flavor is taking over the soup. First, use a slotted spoon to remove as much celery from the pot as possible. This won't remove the celery's flavor from your broth, but it does prevent the problem from growing any worse. Next, remove a cup or two of the celery-heavy broth from the pot -- reserve it for another use -- and replace it with chicken broth, either homemade or store-bought. Taste the soup, and repeat the remove-and-replace technique if necessary. If the broth was unusually rich, you can often use water for the first substitution without harming your soup.
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Bulk It Up
Increasing the size of your batch is a second option. If your soup is still at an early stage, add more chicken and onion and several cups of water. Return the pot to a simmer, tasting the broth frequently as the new pieces of chicken cook and lend their flavor to the pot. When you're satisfied that the flavors have returned to their proper balance, finish the soup by removing and dicing the chicken pieces and then returning them to the pot. Add noodles, herbs, grains or other ingredients, as called for in the recipe.
Go Head to Head
If you lack the time or ingredients to truly fix your celery problem, distracting from its flavor is a viable alternative. Salt works, but it's a high-risk option because it's easy to use too much and create a greater problem. Herbs are usually a better option, since many of them go well with chicken. Simmer a sprig of rosemary in the broth for a few minutes, for example, then remove it once its evergreen flavor overshadows the celery. Fresh sage is equally emphatic, and it's a canonical companion to chicken's flavor. Similarly, a sprinkling of chopped tarragon gives the soup a plausibly Mediterranean flavor. If no green herbs are available, add garlic in small increments until the celery fades from the foreground.
The Ounce of Prevention
The best technique, of course, is simply to not use too much celery in the first place. Professional chefs work from standard ratios of ingredients, observed and refined by generations of their predecessors. In mirepoix, the classic combination of onions, carrots and celery used to flavor broth, the ratio is quite simple. Onions should make up half of the total, with the remainder divided equally between carrots and celery. Too much celery makes the broth bitter, while too much carrot makes it sweet. For a good-flavored soup, use a pound of chicken for every quart of water, and about 12 ounces of mirepoix total.