Kaffir lime leaves are one of the rare ingredients that adds elusive flavors to Thai dishes.

The leaves of a lime tree, they're usually shredded or torn before being used, to help release the volatile oils carrying their flavor.

They add a floral, citric note to soups and curries. Sliced as thin as possible, they're sometimes used in Thai stir-fries.

The kaffir lime fruit is bumpy, with a highly aromatic zest. But the juice is so acidic it's sometimes used as a cleaner, instead of in cooking.

Secret stash: If you can find kaffir lime leaves at an Asian grocery or supermarket, freeze a few small bags. (I got mine at An Chau, 3306 Bailey Ave.) They'll keep for months that way, and remain effective flavoring agents.

Home grown: Some varieties of kaffir lime can be grown in pots in northern climates, though they must be brought inside if frost threatens.

Here, kaffir lime leaves add aroma and a background citrus note to chicken coconut soup, or tom kha gai. By all means still make the soup if tracking down kaffir lime leaves proves onerous. It'll still be delicious.

> Chicken coconut soup (tom kha gai)

1 (14-ounce) can coconut milk, preferably Chaokoh

4 cups chicken broth

1 1-inch piece fresh ginger, peeled and matchsticked

4 scallions, chopped

1 stalk lemongrass (optional)

1 pound thinly sliced chicken breast

1 cup sliced shiitake mushrooms

2 tablespoons fresh lime juice, or more to taste

1 tablespoon fish sauce, or more to taste

1 teaspoon sugar, or more to taste

1 teaspoon Thai red curry paste, or sambal oelek

1 tablespoon chopped cilantro stem

1/4 cup chopped cilantro leaves

2 kaffir lime leaves, torn (optional)

Trim and peel lemongrass, discarding top two-thirds. Bruise remaining core with flat of knife and chop into 1-inch lengths.

In a medium saucepan, combine coconut milk, broth, ginger, scallions, cilantro stem, kaffir lime leaves and lemongrass and bring to boil over high heat.

Lower heat to simmer. Add chicken and mushrooms, and cook until chicken is done, about 5-7 minutes.

Add fish sauce, lime juice, cilantro leaves and sugar. Simmer 5 minutes. Taste and add lime juice, fish sauce and sugar as desired for sweet-sour-salty balance.

Ladle into bowls and top with more chopped cilantro.

Note: Like bay leaves, chunks of lemongrass and kaffir lime leaves aren't usually eaten, just left behind in the bowl.