Preheat the oven to 425 degrees F.Place the squash on a plate and microwave 3 minutes. Let cool slightly, then cut in half lengthwise, scoop out the seeds and discard. Place the squash in a baking dish and season the cut sides with salt and pepper. Transfer to the oven and bake 30-35 minutes or until the squash is tender. Meanwhile, heat 6 tablespoons butter, the walnuts, sage, and a pinch of salt in a large skillet. Cook 5 minutes, stirring occasionally, until the butter is browned and the walnuts are toasted. Remove the nuts from the skillet to a plate. In the same skillet, melt the remaining 2 tablespoons butter with the garlic. Cook until the garlic is fragrant, about 2 minutes. Add the crushed red pepper flakes, milk, goat cheese, manchego, and pesto. Season with salt and pepper. Bring to a gentle simmer over medium heat, stirring constantly until smooth and creamy. Remove from the heat. When the squash is ready, use a fork to scrape the squash into strands, then toss the squash with the pesto cream sauce. Serve the squash warm, topped with buttery walnuts.