Using a sharp paring knife, peel the skin and bitter white pith from the grapefruits. Working over a bowl, cut in between the membranes to release the grapefruit sections. Squeeze the juice from the membranes into the bowl; there should be 1/2 cup.
In a bowl, mix the onion, soy sauce and lemon juice with 4 tablespoons of the oil. Stir in 1/4 cup of grapefruit juice from the bowl and season with salt. Let stand for 10 minutes.
In a skillet, heat the remaining 2 tablespoons of oil until shimmering. Add the crab and cook over high heat, turning once, 30 seconds. Add the remaining 1/4 cup of grapefruit juice and cook until the crab is lightly caramelized, about 10 seconds. Remove from the heat.
Arrange the grapefruit in shallow bowls. Layer the avocado on top, then the crab. Spoon the dressing over the crab, garnish with the almonds and cilantro and serve.