Slice the spaghetti squash lengthwise and scoop out the seeds. Place the two halves face-down on a baking sheet and bake for 30 minutes.
While the squash is cooking, get the beef going. Heat a large pan over medium heat and add in the ground beef. Break it into pieces with a spatula and cook, stirring occasionally, until browned and cooked through, about 8 minutes. Remove from pan and set aside. Reserve at least 1 tbsp of fat in the pan for the sauce you’ll make later.
Remove squash from the oven after 30 minutes and set aside to cool. Keep the oven on at 425°.
While the squash is cooling, prepare the creamy garlic sauce. Whisk together coconut milk and arrowroot flour in a bowl or jar and set aside.
Heat the same pan you cooked the beef in over medium heat. Once hot, add mushrooms and cook until they begin to soften about 2 minutes. Add in minced garlic and mix well with the mushrooms, about 1-2 minutes, until fragrant.
Next, whisk up the coconut milk mixture again and add to the pan, stirring constantly for 2 minutes. The sauce will bubble and thicken, keep stirring to prevent burning. After 2 minutes turn the heat off and stir in salt and pepper.
Now, put it all together. With a fork, scrape out the spaghetti squash “noodles” into a medium casserole dish. Add the beef, broccoli, and creamy garlic sauce. Mix everything together well.
Place back in the oven to bake for 15 more minutes. Remove and serve.