Arrange racks in upper and lower thirds of oven; preheat to 425°. Scoop out flesh from squash. Cut flesh 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.