Cut the squash in half and scoop out the guts and seeds with a spoon; place them in a broiler pan and drizzle with a generous amount of extra-virgin olive oil, then sprinkle liberally with salt and pepper.
Turn the squash flesh side down and bake in the oven, uncovered, for about 60-75 minutes, until softened and the flesh can easily be pierced with the tip of a sharp knife.
About 10 to 15 minutes before your squash is done baking, melt the ghee in a skillet set over high heat and add the mushrooms. Cook them until nice and golden brown, then lower the heat to medium, add the onion and let it sweat for 3-4 minutes, until softened and translucent.
While the mushrooms and onions are cooking, add the raw cashew butter, Dijon mustard, sage, thyme, salt and pepper to a large glass measuring cup and mix well with a whisk until completely combined. Add the water, about ¼ cup at a time and mix well between each addition, to avoid the formation of lump and ensure that the sauce will be smooth and creamy.
Pour that cream over the cooked mushrooms and onions, turn the heat down to low and mix until well combined.
Throw in the chopped spinach and delicately mix them in. Kill the heat and let the sauce sit for a few minutes while the spinach wilts, then give it a final stir. If you feel the sauce is too thick, feel free to add a little bit more water.
When your squash is done baking, remove it from the oven and let it cool for a few minutes, then gently scrape the flesh with a fork so to form strands that resemble spaghetti.
Transfer those strands to shallow bowls or plates and top with the creamy mushroom sauce.