Preheat oven to 400 degrees F (200 degrees C).
Halve the Spaghetti Squash (1) and remove the seeds.
Place halved spaghetti squash cut-side down on baking sheet. Bake for 35 to 45 minutes or until fork tender. Or you can place in a large pot with and 1-inch of water and steam till fork tender.
Remove the cooked squash from the oven or pot, melt the Butter (1 tablespoon) in a skillet over medium heat and saute the Yellow Onion (1/2) and Garlic (1 clove) till tender and slightly golden.
Transfer the cooked onion and garlic to a large mixing bowl, and combine with the Greek Yogurt (1 cup) and Shredded Cheese (2 ounce), and season with Salt (to taste) and Ground Black Pepper (to taste).
When the spaghetti squash is cool enough to handle, use a fork to scrape the noodle like strands into the bowl, mix well.
Transfer the mixture to a roughly 2.5 quart casserole dish and smooth the top with a spatula. Sprinkle the remaining shredded cheese over the top, and return to the oven to bake for 30 minutes at 400 degrees F (200 degrees C).
The casserole is ready when the top is a light golden brown.