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Adjust rack to top position and preheat oven to 425 degrees. Wash and dry produce.
Halve, peel, and thinly slice onion; mince a few slices until you have 2 TBSP (4 TBSP for 4 servings). Cut potatoes into ½-inch-thick wedges. Halve buns.
In a small bowl, combine mayonnaise with chipotle powder to taste (start with a pinch, then taste and add more from there if desired). Season with salt and pepper.
Cut broccoli into bite-size pieces if necessary. (Save potatoes for another use.)