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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 ¼ of the onion; dice remaining onion. Quarter lime. Halve poblanos lengthwise; remove stems and seeds. Peel and mince garlic.
In a small microwave-safe bowl, combine sliced onion, juice from half the lime (whole lime for 4 servings), ¼ tsp sugar (½ tsp for 4), and a pinch of salt. Microwave until onion is bright pink, 30 seconds. Set aside.