Cover the cashews with boiling water and soak for 15 minutes, then drain.
Bring a medium pot of well salted water to a boil, using a little less water than usual so it gets extra starchy. Cook the spaghetti until 1 to 2 minutes shy of al dente, reserving 1 1/2 cups of the pasta water before draining.
While the pasta cooks, scoop out 3/4 cup of the hot starchy water and blend it with the cashews, nutritional yeast, miso, lemon juice and salt until completely smooth and pourable.
Melt the dairy free butter in a large skillet over medium heat. Add the cracked pepper and toast for about 1 minute until very fragrant.
Add the spaghetti and 1/4 cup of the remaining pasta water to the skillet and toss for 1 minute to finish cooking.
Remove the pan from the heat and pour in the cheesy sauce. Toss vigorously with tongs, adding pasta water a splash at a time, until a glossy, creamy sauce coats every strand. Serve immediately with extra cracked pepper.