Cook the Spaghetti Squash:

Cup of Yum
1. Cook squash according to your desired preference. Oven Directions: Cut squash in half and remove the seeds. Drizzle with oil and a pinch of salt (if desired). Roast cut-side down on a baking sheet at 400°F for about 40 minutes, or until the squash is easily pierced with a fork.Pressure Cooker: See Instant Pot directions.Allow squash to cool slightly. Use a fork to scrape the spaghetti squash out of the peel.

Make the Casserole:

1. Preheat oven to 400°F and move an oven rack to the top position (near the broiler).Heat 2 TB olive oil over medium heat in a large, oven-safe pan (like cast iron or carbon steel).Add the spaghetti squash, garlic, salt and red pepper flakes. Cook about 3 minutes to remove some of the moisture from the squash.
2. Add the flour to the squash mixture and stir. Cook about 2 minutes until the flour is incorporated. Stir well and make sure there are no lumps of uncooked flour before proceeding.
3. Slowly pour the milk into the pan, stirring as you pour. Continue cooking until the liquid begins to simmer and the milk begins to thicken.Simmer for about 2 minutes (or until the milk begins to thicken), and then stir in ½ cup of the cheese. Make Ahead Tip: If desired, you can stop at this point and finish cooking later. Make up to one day ahead, and then reheat to a simmer before proceeding to the next step.
4. Sprinkle the remaining cheese across the top of the squash and drizzle with the remaining 1 TB oil.
5. Carefully move the pan into the oven onto the top rack. Bake 13 minutes. Position the pan so that it's directly under the broiler. Turn on the broiler and cook for 2 more minutes, until the cheese is golden. (If your oven does not have a broiler, simply cook 2 more minutes.)Serve immediately.