Cut you spaghetti squash in half and scoop out the seeds with a spoon. Bake cut side down on a baking sheet for 45 minutes-1 hour or until tender. Alternatively, you can microwave for 8-10 minutes.
Using two forks, scrape the "noodles" out of the spaghetti squash and set the shells aside for later. Place the squash noodles in a paper towel-lined colander to drain the excess moisture.
Meanwhile, melt butter in a large pan. Add the mushrooms, onion, and garlic, and cook till tender, then remove from the pan and set aside.
Heat the olive oil in the same pan and add the beef, and cook until browned on all sides.
Add 1 cup of the broth, salt, and Worcestershire sauce to the beef in the pan.
Bring to a boil, cover, reduct heat and simmer for 15 minutes.
Whisk together the remaining half cup of broth and cornstarch. Stir the slurry into the beef mixture in the pan, add back the mushroom mixture, and bring to a boil. Let boil for 1-2 minutes, or until thickened.
Turn off the heat and stir in the Greek yogurt.
Place the squash shells in a baking dish. Divide the squash noodles between the shells and make a well in the center. Divide the beef mixture between the shells and sprinkle with parmesan cheese.
Bake at 400°F for 10-12 minutes, or until heated through and golden brown.