To Prepare 1 Large Spaghetti Squash:
Preheat the oven to 400 degrees. Cut the squash in half, remove the seeds (with a spoon or ice cream scooper), and bake on a baking sheet for about 25-45 minutes until tender (you should be able to pierce through the flesh easily). Note that baking times may vary based on the size and thickness of your squash. If you over-bake, the strands of spaghetti squash will become mushy (potato-like) when you scrape them out, so be sure not to over-bake them.
Using a spoon, start to scoop out and separate the flesh of the spaghetti squash until all that remains is the outer shell. You’ll notice that the flesh of the squash comes out in strands similar to spaghetti (hence the name “spaghetti squash”).
To Prepare the Baked Spaghetti Squash Dish:
Preheat the oven to 375 degrees.
In a large sauce pan, heat the butter and olive oil on medium heat.
Add the onions and garlic and cook for a few minutes until fragrant.
Add the flour and whisk to combine. Reduce the heat to medium-low and cook for a few minutes, stirring the flour/butter mixture occasionally.
Slowly add the milk and whisk continuously on medium heat until the mixture becomes smooth and starts to simmer. Continue to simmer, stirring frequently, until the sauce thickens in consistency. Make sure the sauce is thicker than a standard bechamel sauce. As a guide, the sauce is thick enough right when the sauce just starts to clump (just prior to this moment). In other words, the sauce should not pour out in a uniform stream. (see notes below)
Remove from heat and stir in the shredded cheddar cheese. Combine until smooth (i.e. the cheese should melt into the sauce).
Season with salt and pepper.
Add 5 1/2 cups cooked spaghetti squash and spinach leaves and combine.
Transfer everything to a baking dish (such as a 9×13 glass tray) and sprinkle the top with grated Parmesan Cheese.
Bake for 40-45 minutes until the dish is bubbly and the top is golden. Note that if you do not bake for long enough, the dish may be slightly runny because the water released from the spinach did not have enough time to boil off.
Remove from the oven and let it sit for 5-10 minutes to allow the dish to finish cooking. Sprinkle some fresh ground black pepper (optional) and serve immediately. Enjoy!