Cut the spaghetti squash in half lengthwise. Be careful as it is tough to cut through. Place the squash on towels if necessary to keep it in place.
Remove the seeds by scraping the inside of each half with a spoon.
Drizzle the inside of each half with the vegetable oil and sprinkle with salt and pepper.
Place the squash cut side down on a baking sheet. Roast in the oven until tender when pierced with a knife, approximately 30-40 minutes. Remove from oven and let cool slightly to handle.
While the squash is cooking, cook the chicken (if not cooked already) as preferred. I just drizzle with oil, salt, pepper, and garlic powder and cook in the oven while the squash is cooking. Leftover chicken is perfect for this as well. Remove the chicken when done and cut into strips.
Fluff up the interior of each squash by using a fork and shredding at the squash flesh. This will make the "noodles"!
Add 1/2 cup of the pesto to each squash boat and mix to combine with the squash.
Top each squash boat with the chicken (or toss it throughout the squash with the pesto in the step above).
Sprinkle the cheese across the top of each squash boat, over the squash and chicken. Bake for approximately 15 minutes until the cheese is melted. If you want browned cheese, you can pop the squash under the broiler once the cheese is melted (just make sure it is no longer on parchment or silicone or anything else that would burn!).
Drizzle with some additional pesto if desired. Serve immediately.