Cut spaghetti squash in half, remove seeds and place each half cut side down on a sheet pan. Place sheet pan in preheated oven and roast spaghetti squash until tender, about 20-25 minutes. Squash is ready when you can press the outside (careful) and it "gives" and is no longer hard.
Remove spaghetti squash from oven and place cut sides up, allowing to cool slightly.
While spaghetti squash is roasting, place onion, peppers, tomatoes, zucchini, yellow squash, and broccoli on another sheet pan. Add granulated garlic, Italian seasoning and salt. Pour on olive oil and toss everything together to evenly coat.
Roast vegetables in 400 degree oven for about 15-20 minutes, while spaghetti squash is roasting. Vegetables are ready when tender-crisp and golden brown. Remove from oven.
Using a fork, shred the spaghetti squash into "spaghetti" and place in a large mixing bowl or casserole dish.
While the spaghetti squash is still slightly warm, add the chopped baby spinach and roasted vegetables. Toss to blend thoroughly. Taste for seasoning, adjust if necessary and serve.