Mix salt and sugar, and sprinkle over fish, tossing to coat. Cover with plastic wrap and refrigerate for one hour. Remove the fish from curing mixture and rinse under cold water. Transfer to paper towels and pat dry.

Dice fluke into half-inch cubes. In a small bowl, combine yogurt, lime juice and zest, and olive oil. Season with salt and pepper to taste. Dollop on a platter, then spoon fish over yogurt sauce. Arrange serrano and cucumber alongside fish, then sprinkle with cashews and salt. Finish with cilantro, microgreens, and a drizzle of olive oil just before serving.