Preheat oven to 450°F with a rack in the lowest position. Toss sliced squash, 2 tablespoons oil, ½ teaspoon pepper and ⅛ teaspoon salt on a large rimmed baking sheet; spread in a single layer.
Roast until well browned on the underside, 25 to 30 minutes, rotating the pan from front to back halfway through. Flip the squash and continue roasting until lightly browned on the other side, 10 to 15 minutes. Sprinkle with ⅛ teaspoon salt.
Meanwhile, whisk 3 tablespoons yogurt, 1 tablespoon mayonnaise, 1½ tablespoons sweet chili sauce, 1 tablespoon Sriracha, 1 teaspoon onion powder and the remaining ¼ teaspoon salt in a small bowl until smooth.
Spread the sauce in an even layer on a large platter. Top with the roasted squash and sprinkle with 2 tablespoons scallions and ½ teaspoon sesame seeds.
