Heat oven to 400º F. Place the salmon on a rimmed baking sheet and drizzle with 1 tablespoon of the oil.
Rub the salmon with the brown sugar, ancho chili, and ½ teaspoon salt. Roast until opaque, 12 to 15 minutes.
Meanwhile, heat the remaining tablespoon of the oil in a large skillet over medium-high heat.
Add the squash, ½ teaspoon salt, and ¼ teaspoon pepper and cook, tossing frequently, until just tender, 5 to 7 minutes; fold in the scallions. Serve with the fish.