Preheat oven to 400 degrees. Cut spaghetti squash in half, then scoop out the seeds; discard. Spray the inside of both squash with cooking spray, then place cut-side up on a foil-lined cookie sheet.
Roast squash until tender when pierced with a knife, about 45-50 minutes.
Remove squash from the oven and let cool about 10 minutes. Once it has cooled down, scrape the flesh into a large bowl.
In a large skillet, melt the butter over medium heat; add flour and whisk constantly for about 30 seconds. Add the milk and whisk together.
Reduce the heat to low, then add the four cheeses, one at a time, making sure to incorporate the cheese completely before the next addition.
Add the spaghetti squash to the cheese mixture and mix. Season with salt, pepper and red pepper flakes. Serve; garnish with fresh parsley, if desired.
Recipe by sweet caroline's cooking at https://sweetcarolinescooking.com/2015/10/16/wont-miss-a-thing-four-cheese-spaghetti-squash-mac-cheese/