Herbs are supposed to contribute both taste and aroma to a long-cooked dish. Unfortunately, herb's fragrance is ephemeral - it doesn't take long for the heat to dissipate the herb's precious volatile oils. That's why it's wise to add aromatic herbs in two stages to a long-cooked dish. First, coarsely chop half the herb supply you plan to use. Stir it in the pot at the beginning of the cooking process, but after all the cooking liquid has been added. The delicate aromatics of this batch of herbs will be lost, but the herbs will have an hour or two to interact chemically with other ingredients, creating new flavors that will add taste complexity to the dish. Fifteen minutes or so before the dish is cooked and served, finely chop and add the remaining herbs - their flavor and fragrance will be noticeable to the diner.
** Asian Recipes