Preheat your oven to 425 degrees F and line a large baking sheet with foil.
Cut the squash in half, lengthwise using a large sharp knife. Scoop out the seeds and strings, then place face-down on a foil lined baking sheet.
Roast in the oven for about 25 minutes or until the squash can be pushed from the outside a bit with your finger.
Allow the squash to cool enough to handle, and, using a fork going lengthwise down the squash, remove spaghetti strings from both sides of the squash and place in a large bowl.
In a casserole dish, combine the spaghetti squash with the pesto sauce. Toss gently to coat. Place in the oven for 5-10 minutes to heat through.