To prepare the beurre blanc, first finely mince the lower third of the lemongrass stalk. Next, finely mince the ginger. Cut the cold butter into tablespoon-size pieces and set aside. Gather the rice wine vinegar, white wine and lemon or lime (if using).
Place a small saucepan over medium heat. Add the lemongrass, ginger, rice wine vinegar and white wine. Bring to a simmer and reduce the liquid by about two-thirds until it reaches a syrupy consistency.
Turn the heat to the lowest setting and whisk in the cold butter one piece at a time to slowly form the emulsion.
Once all of the butter has been incorporated, season with salt and pepper. You may want to add a few drops of lemon or lime juice to brighten the flavor of the sauce.
Monitor the sauce closely while you cook the chicken. Keep the sauce warm to the touch and whisk often to prevent it from splitting. You may need to turn the heat off and on to keep it at the correct temperature. The heat does not always need to be on, as the residual heat from the pan will keep it warm.