Preheat the oven to 400°F (200°C).
Halve the spaghetti squash lengthwise and scoop out the seeds; place each half cut-side down on a sheet pan and roast until tender, about 20–25 minutes.
While the squash roasts, spread the diced onion, diced red peppers, halved grape tomatoes, diced zucchini, diced yellow squash, and broccoli florets on a second sheet pan.
Sprinkle the vegetables with granulated garlic, Italian seasoning, and salt, drizzle with olive oil, and toss to coat evenly.
Roast the vegetables in the 400°F oven for about 15–20 minutes, until tender-crisp and lightly browned; remove from the oven.
Let the spaghetti squash cool slightly, then use a fork to shred the flesh into strands and transfer them to a large mixing bowl or casserole dish.
While the squash is still warm, add the roughly chopped baby spinach and the roasted vegetables to the bowl with the shredded squash.
Toss everything together until combined, taste and adjust seasoning if needed, and serve warm.
