In a small saucepan bring 2 cups of water to a simmer. Remove from heat.
Add shrimp to saucepan. Stir and cover. Cook shrimp for about 2-3 minutes or just until opaque. Drain and set shrimp aside to cool.
While shrimp is cooling prepare salad ingredients. Segment pomelo or grapefruit by cutting the top and bottom off to expose just the top of the fruit’s flesh. Set the fruit on a cut end. Working from top to bottom, and following the contour of the fruit, cut away the peel and pith, leaving only the flesh.
Once the peel and pith are removed, slide a paring knife along each of membranes to remove the fruit segments.
In a large bowl, combine segmented citrus, toasted cashews, cilantro, shallots, and cucumber. Reserve some cashews and cilantro for garnish.
Once cool, slice shrimp in half lengthwise and add to salad mixture.
In a small bowl, combine fish sauce, lime juice, sugar, pepper, and garlic. Stir until the sugar dissolves. Pour over shrimp and salad mixture and gently toss to combine.
Divide mixture into lettuce leaf cups. Top with cilantro and cashews for garnish. Serve.