Preheat the oven to 425. Carefully cut the spaghetti squash in half. Scoop out the seeds and drizzle with olive oil. Transfer to a tin foil lined baking sheet with the cut side of the squash down.
Bake for 40-45 minutes, until the squash is tender. If you are short on time, see the text above for instructions on microwaving the squash.
Let the squash cool for a few minutes. While it is cooling slightly, add the butter to a pan over medium heat. Add sliced mushrooms and cook down for 5 minutes. Take off the heat.
Using a fork, scrape the inside of the cooked squash and it will come off in spaghetti shaped pieces. Transfer to a bowl and add the pesto. Toss until the squash is fully covered.
Divide the squash between two plates and top with mushrooms, parmesan cheese, and pine nuts. Enjoy!