Japanese parsley, or mitsuba, is traditionally used as a garnish in miso soup, stir-fry and other traditional Japanese dishes. It looks like Italian flat-leaf parsley, but its flavor is very distinct. If your local grocery store does not carry Japanese parsley, you may be able to find it in smaller specialty groceries or online. If Japanese parsley is not available in your area or is out of season, you can use other herbs to approximate its flavor. You will not be able to achieve a perfect substitute for traditional mitsuba using other herbs, but you can come close.
Watercress imparts a deep peppery flavor to any dish that features it. Slice the leaves into ribbons and use in place of Japanese parsley in soups and stir-fries. Its intense aroma stands up to high heat and competing flavors. When substituting watercress for Japanese parsley, use about half as much to avoid overwhelming the other flavors in the dish.
The leaves of the celery plant have a mild flavor and delicate texture, but do not have the depth of flavor that you can get from Japanese parsley. Cut the leaves into ribbons or leave smaller ones whole before using them to garnish miso or other soups.
Angelica is a delicate herb with a celery-like flavor. Its crunchy stems make it ideal for stir-fry and rice dishes, where it gives a textural contrast to the cooked vegetables. When using it in a stir-fry, add it after you remove the dish from the heat to avoid wilting, or offer it as a garnish at the table.
No one herb is an exact substitute for Japanese parsley. If you want to get as close as possible to the original flavor, try mixing two or more herbs. For example, a blend of watercress and celery leaves or angelica will give a close approximation to the original flavor of Japanese parsley. Mix chervil, Italian parsley, and arugula for a milder, less celery-like take on Japanese parsley.