Preheat oven to 375 degrees
If roasting spaghetti squash in the oven, cut the squash horizontally and scoop out the seeds. Pour 2 inches of water in a baking dish, then place the cut side of the squash in the pan and cook for 35-45 minutes.
If making the spaghetti squash in the microwave, score the squash horizontally with a sharp knife - cutting roughly ½ deep. Spear small cut marks around the entire squash then microwave for 5 minutes
Using oven mitts, remove the squash from the microwave and finish cutting the squash in half. Scoop out the seeds. Place cut side of the squash in a baking dish with 2 inches of water and microwave for another 5-7 minutes (you may need to do each side separately to fit into the microwave). Shred the softened squash with a fork to create spaghetti-looking noodles
In a 9x13 baking dish, add the shredded squash, chicken, drained sun-dried tomatoes, and basil mixture. Sprinkle with salt and pepper
For the Sauce:
In a saucepan whisk together all the ingredients for the cream sauce. Heat the sauce on the stove over medium heat. Continue whisking the sauce while it cooks for about 5-8 minutes until it thickens.
Then pour the cream sauce over the casserole and bake for 30 minutes. Serve immediately or let cool and refrigerate to reheat later.