Step 1

Arrange racks in upper and lower thirds of oven; preheat to 425°. Scoop out seeds from squash. Cut squash lengthwise into 1" strips, then crosswise into 1" pieces. Toss squash, cauliflower, onion, and 3 Tbsp. oil on a large rimmed baking sheet; season with salt. Roast on top rack, tossing occasionally, until vegetables are browned all over and tender, 25–30 minutes.

Step 2

Reduce oven temperature to 300°. Drizzle salmon lightly with oil; season all over with salt. Push vegetables to one side of baking sheet, then place salmon on other side. Spread pumpkin seeds on a small rimmed baking sheet. Bake salmon and vegetables on lower rack until fish is just firm in the center and opaque, and bake pumpkin seeds on top rack until golden, 12–16 minutes.

Step 3

Meanwhile, whisk orange juice, lime juice, vinegar, miso, and remaining 2 Tbsp. oil in a small bowl until miso dissolves. Stir in chiles; season with salt.

Step 4

Break salmon into large pieces. Arrange vegetables on a platter, then top with salmon. Drizzle with dressing and scatter pumpkin seeds over.

Step 5

Do Ahead: Dressing can be made 2 days ahead. Cover and chill.