To marinate the onion, cut it crosswise into slices ⅛ inch (3 mm) thick. Place the slices in a small bowl and toss with the raspberry vinegar, a pinch of salt, and sugar. Let stand for about 30 minutes to soften and mellow the onion. Drain.

Meanwhile, place a collapsible steamer rack in a saucepan with about ½ inch water

(the water should just come up to the bottom of the rack) and bring to a simmer over medium-high heat. Add the shrimp, cover, reduce the heat to medium, and steam the shrimp until evenly pink, about 2 minutes, stirring once if necessary. Remove from the heat and let cool to the touch. Peel and devein the shrimp, leaving the tail segments intact if desired. Zest one of the oranges, then peel and segment all of them, reserving the juice.

To make the dressing, in a small bowl, combine the oil, sherry vinegar, the ½ Tbsp lemon juice, and 1 Tbsp of the reserved orange juice, season to taste with sea salt and white pepper, and whisk to blend. Taste and adjust the seasoning, adding a little more lemon juice if desired.

In a large bowl, combine the watercress, lettuce, and cilantro sprigs. Add all but 2 Tbsp of the dressing and toss to combine. Transfer the salad greens to a large, shallow bowl or platter or 4 individual bowls. Arrange the crab, shrimp, orange segments, and avocado slices on top and drizzle the remaining dressing over them. Scatter the marinated onion over the salad and sprinkle with the orange zest. Serve at once.

VARIATION TIP: Replace the oranges with 1 large mango; cut the flesh from the pit and then peel the halves and cut them into slices. Replace the orange juice in the dressing with fresh lemon juice.