Preheat your oven to 400°F.
Cut the spaghetti squash in half lengthwise and scoop the seeds out.
Place the squash cut side down in a baking dish and add about ¼ inch of water. Bake until tender - about 45 minutes. You'll know it's ready when you can easily pierce the outer skin and inner part with a fork.
While you're waiting on the squash to cook, melt the butter over medium heat in a large pan and add the garlic, saute for 2-3 minutes, then add the cream and bring to a simmer. While the cream is simmering, slowly whisk in the cheese and dried herbs, then continue simmering on low for 5-7 minutes, then sprinkle arrowroot over the top while whisking constantly. Turn off the burner but leave the pan where it is. The sauce will continue to thicken as it slowly cools, so don't worry if it's a bit thin at first.
When the squash is ready, remove it from the oven. Scoop out the squash and place it in a bowl, pouring off any excess water that collects at the bottom. Pour over the alfredo sauce and mix together. Add salt to taste (if needed) and garnish with fresh parsley or basil and extra Parmesan-Reggiano if desired, and serve.