Preheat oven to 400°F. Cut neck portion off squash. Peel and quarter neck lengthwise. Cut crosswise into 1-inch-thick pieces. Peel and seed rounded end of squash; cut into 3/4-inch-wide wedges. Place squash in very large bowl; add onions, fennel, and Jerusalem artichokes. Place beets in separate large bowl. Add 2 1/2 teaspoons thyme, 2 teaspoons rosemary, and 2 tablespoons oil to beets; sprinkle with salt and pepper and toss to coat. Add 2 tablespoons plus 1/2 teaspoon thyme, 1 tablespoon plus 2 1/2 teaspoons rosemary, and 6 tablespoons oil to bowl with squash; sprinkle with salt and pepper and toss to coat.
Spray rimmed baking sheet with nonstick spray. Scatter beets on sheet. Roast until tender, about 45 minutes.
Spray 2 large rimmed baking sheets with nonstick spray. Divide remaining vegetables between sheets. Roast until vegetables are tender, stirring occasionally, about 1 1/2 hours.
Transfer all roasted vegetables to platter. Serve warm or let stand up to 3 hours at room temperature.
*Jerusalem artichokes are tubers of a variety of sunflower; available in the produce section of some supermarkets and at farmers’ markets.