Whisk fish sauce and sugar until the sugar is dissolved. Stir in lemongrass, galangal and lime leaves. Marinate: ½ to 1 hour.
Notes: Galangal is a fleshy root used as a seasoning in Southeast Asian cooking and is similar in appearance and flavor to ginger. Look for it fresh in specialty and Asian markets. Store unused portions in the refrigerator or freezer.
Kaffir lime leaves (or makrut or Thai lime leaves) lend one of the signature flavors to Thai cooking—lemony and floral. Like other Asian ingredients, there is no real substitute, but lime zest will work in a pinch. Find the leaves fresh or frozen in Asian markets and online stores. Fresh leaves may be frozen, airtight, for up to 3 months.