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Place a foil-lined baking sheet on top rack (for 4 servings, use 2 foil-lined sheets on top and middle racks) and preheat oven to 450 degrees. Wash and dry produce.
Halve, peel, and thinly slice onion. Drain pineapple over a small bowl, reserving juice. Stir ¼ of the onion, half the vinegar (you’ll use the rest later), and a pinch of salt into bowl with juice. Set aside.