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Adjust rack to middle position (top and middle positions for 8 servings) and preheat oven to 425 degrees. Bring a large pot (two large pots for 8) of salted water to a boil. Wash and dry produce.
Trim and thinly slice scallions, separating whites from greens. Halve, peel, and thinly slice onion. Halve, core, and thinly slice bell pepper.
Generously coat a 9-by-13-inch baking dish (two 9-by-13-inch baking dishes for 8) with nonstick cooking spray and set aside.