Cook the spaghetti squash (see notes for instructions) and pull the "meat" out with a fork, scraping the shell to get it all. Set aside.
In a skillet, melt the butter and add garlic, milk and half & half. Bring to a simmer.
Remove from heat and add the Parmesan cheese and egg yolk. Stir constantly until the cheese melts and yolk is well incorporated. Let sit a few minutes to thicken.
Add the squash and stir to coat.
Serve.
Notes
To cook the spaghetti squash: Carefully cut in half lengthwise. Scoop out seeds and place face down in a baking dish. Poke skin with a fork all over. Add ½ cup water to dish. Bake at 425 degrees F for 40 minutes or until squash "meat" is tender. Turn squash over and use a fork to scrape the meat out.
Recipe by Mandy's Recipe Box at https://mandysrecipeboxblog.com/spaghetti-squash-alfredo/