Preheat the oven to 425ºF. Add the butternut squash into a bowl and top with olive oil and pepper. Toss to coat.
Transfer the squash onto a baking sheet and bake for 25 - 30 minutes until the squash is tender.
Meanwhile, add the walnuts into a small bowl. Top with hot water and allow to soak for at least 20 minutes. Once soaked, drain and add to a blender.
Along with the walnuts, add the nutritional yeast, miso paste, garlic, shallot and water into the blender as well. Blend on high until creamy.
With 10 minutes left in the cooking time on the squash, start the pasta. Cook according to package instructions and in the last 30 seconds of cooking, remove 2 - 3 tablespoons of pasta water and add the spinach. Give it a stir until the spinach is wilted.
Drain the pasta and spinach in a colander, then transfer it back into the pot. Add the roasted squash, 2 tablespoons of pasta water and the sauce. Also, add the lemon juice and fresh sage and toss to combine.
Serve immediately and garnish with a touch more sage, pepper, and crushed walnuts.