Preheat oven to 400℉. Slice spaghetti squash lengthwise, and scoop out seeds.
Place both halves face-down onto a baking sheet; bake 25 minutes.
While the squash is cooking, heat a large pan over medium heat and add the sausage. Cook until browned and crumbly, about 8 minutes.
Remove from pan, and set aside. Reserve at least 1 tablespoon of fat in the pan for the sauce.
Remove spaghetti squash from oven, and set aside to cool.
Prepare sauce by whisking together coconut milk and cornstarch.
Heat the same pan you cooked the sausage in over medium heat; add mushrooms, and cook until they begin to soften. Stir in garlic, and cook about 1-2 minutes, until fragrant.
Whisk the coconut milk mixture again, and add to pan, stirring constantly 2 minutes. The sauce will bubble and thicken. After 2 minutes turn off stove, and stir in salt and pepper to taste.
With a fork, scrape out the spaghetti squash "noodles" into a medium casserole dish. Stir in the cooked sausage, broccoli, and creamy garlic sauce; mix well. Bake an additional 15 minutes or until bubbly.