This very pungent sauce (mắm nêm) is used in central and southern Vietnam. Anchovy fish sauce is very salty, so I recommend diluting it with sugar and lime juice.
Combine the pineapple and water in a blender. Blend well and transfer to a medium bowl.
Gradually add the anchovy fish sauce and adjust to your taste. The more sauce you add, the saltier the flavor. If needed, add some sugar and lime juice to taste (if using).
Heat the vegetable oil in a small pan over medium heat. Add the garlic, lemongrass, and chil