1. In a medium saucepan, whisk 1/4 cup of the coconut milk with the curry paste. Stir in the remaining coconut milk, the stock, lemon zest, ginger, lime zest, 2 tablespoons of the fish sauce and the brown sugar and bring to a simmer. Cook the sauce over moderately high heat until reduced by one-quarter, about 15 minutes.
2. Using a slotted spoon, remove the lemon zest, ginger and lime zest and discard. Stir in the lime juice, cilantro, basil and the remaining 1 tablespoon of fish sauce; transfer to a bowl.
Alternatives:Replace the lemon zest with the bottom 4 inches of 1 stalk of lightly smashed fresh lemongrass, and replace the lime zest with 4 kaffir lime leaves that have been lightly crushed.
Applications: Use as a stir-fry sauce or as a poaching liquid for chicken, shrimp and vegetables. Or, toss with Asian noodles or serve over rice.
Make Ahead: The curry sauce can be refrigerated in a glass jar for up to 2 days.