Position an oven rack in the center of the oven and preheat to 425 degrees F.
Toss the squash with the oil, sesame seeds, 1 teaspoon salt and 1/4 teaspoon pepper on a rimmed baking sheet until completely coated. Roast, tossing once halfway through, until the squash is just tender and beginning to brown on the edges, about 30 minutes.
Meanwhile, whisk together the miso, sriracha, honey and 1/4 cup water in a measuring cup.
Remove the squash from the oven, drizzle with the miso mixture and toss to coat. Continue to roast until the squash is tender and browned all over and the miso mixture is beginning to caramelize, about 10 minutes. Top with the scallions.
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