1/2 cup fish sauce
3 tablespoons freshly squeezed lime juice
2 tablespoons light brown sugar
1 teaspoon finely minced garlic
1 teaspoon finely chopped hot green chillies (jalapeno or thai chilis are good for this; use hotter if you prefer)
1 teaspoon finely minced cilantro or parsley

Combine all the ingredients in a small bowl and stir until the sugar is fully dissolved and all the ingredients have mixed. This can be made in advance of cooking the fish, allowing the flavors to combine.