Mix the lime juice, sugar and 100 milliliters (1/2 cup) water in a small bowl, stirring to dissolve the sugar. Taste and adjust the flavors if necessary to balance out the sweet and sour. Add the fish sauce, garlic and chiles. Taste again and adjust the flavors to your liking, balancing out the sour, sweet, salty and spicy. Cover with plastic wrap and set aside at room temperature until needed, up to 24 hours. Just before serving, mix in the cilantro and Vietnamese coriander.
For this sauce, it is worth seeking out a Vietnamese fish sauce, which is much lighter than the Thai version.
Recipe courtesy of Donal Skehan