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Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 450 degrees. Wash and dry produce.
Halve and peel onion; thinly slice one half. Finely chop remaining onion until you have 2 TBSP. (For 4, thinly slice whole onion; finely chop a few slices until you have 3 TBSP). Dice tomato. Roughly chop cilantro. Quarter lime. Halve, core, and thinly slice green pepper into strips.