Cook shallot, garlic, cashews and olive oil in a small saucepan, over medium-low heat, stirring often, until cashews are golden brown and shallots are deep golden brown, 8–10 minutes. Strain through a fine-mesh sieve into a small bowl; save oil for another use. Transfer cashew mixture to a medium bowl. Add sweetener and salt and toss to combine. Let cool.
Blend cashew mixture, lime juice, fish sauce, and ½ cup water in a blender or food processor until very smooth and pourable (it should be about the thickness of tahini).