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Adjust rack to top position and preheat oven to 425 degrees. Wash and dry all produce.
Halve, peel, and thinly slice onion. Zest lime until you have 2 tsp (4 tsp for 4 servings); quarter lime. Dice sweet potato into ½-inch pieces. Roughly chop cilantro. Drain pineapple, reserving juice. Remove and discard any large ribs from kale.
In a medium bowl, combine half the onion, juice from half the lime, ½ tsp sugar (1 tsp for 4), and a pinch of salt. Set aside to quick-pickle.