For the Pickled Onions

1. In a medium skillet, combine the vinegar, sugar, and salt. Bring to a boil, whisking to dissolve the sugar. Add the onion, reduce to a simmer and cook, uncovered, gently stirring, until the onions have wilted slightly, 1 to 2 minutes. Transfer the onions and pickling liquid to a heatproof bowl and let cool.

For the Salad

1. Bring a large pot of salted water to a boil. Add the corn, then cover the pot and turn the heat down to low. Simmer for 10 minutes, then remove the corn from the water and let cool. Holding the cooled corn upright in a large mixing bowl, cut the kernels off the cob in strips.
2. To the bowl of corn, add the beans, scallions, cilantro, 5 tablespoons of the oil, the lime juice and lemon juice, ½ teaspoon of the salt, and the cumin. Toss well and set aside.
3. In a large skillet, heat the remaining 3 tablespoons of oil over medium-high heat until hot but not smoking. Add the shrimp and season with the remaining ¼ teaspoon salt. Cook for 1 minute, then add the garlic and cook, stirring occasionally, until the shrimp are just cooked through, 1 to 2 minutes.
4. Transfer the shrimp to the mixing bowl with the corn, scraping all of the garlicky oil from the pan into the bowl. Toss the salad, then taste and adjust seasoning, if necessary (be sure to taste the salad with the pickled onions and feta; they balance the flavor). Transfer to a serving bowl. Crumble the feta over the salad, gently toss, and sprinkle with extra cilantro and fresh ground black pepper, if desired. Remove the pickled onions from their pickling liquid and serve on the side. The salad may be served room temperature or cold.
5. Make-Ahead: The salad is best served on the same day of assembling, but it can be refrigerated in an airtight container for up to 2 days. Taste and adjust seasoning again before serving.