Preheat and prep the squash. Heat your oven to 400°F (200°C). Carefully slice the spaghetti squash in half lengthwise and scoop out the seeds. Drizzle the cut sides with 1 tablespoon olive oil, and season with salt and pepper.

Roast until tender. Place the squash halves cut side down on a lined baking sheet. Roast for 35–45 minutes, or until the flesh is easily pierced with a fork. The strands should pull apart easily.

Cool and shred. Let the squash cool slightly. Use a fork to scrape the flesh into long strands. Pat dry with paper towels if it looks watery. Set aside.

Sauté the mushrooms. Heat the remaining 1 tablespoon olive oil in a large skillet over medium-high heat. Add the sliced mushrooms with a pinch of salt. Cook, without stirring much at first, until browned and their moisture evaporates, about 6–8 minutes. Transfer to a plate.

Start the Alfredo base. In the same skillet, lower the heat to medium. Add the butter. When melted, add minced garlic and cook until fragrant, about 30 seconds—don’t let it brown.

Build the sauce. Pour in the heavy cream and whisk to combine. Simmer gently for 3–4 minutes to thicken slightly. Stir in the Parmesan and nutmeg, and season with salt and pepper. The sauce should be silky and coat the back of a spoon.

Add mushrooms and squash. Return the mushrooms to the pan and fold in the spaghetti squash strands. Toss gently to coat everything in the sauce. Warm through for 2–3 minutes. If the sauce gets too thick, splash in a little more cream.

Finish and serve. Taste and adjust seasoning. Sprinkle with extra Parmesan, chopped parsley, and red pepper flakes if you like. Serve hot.