Process cashews in a blender on low speed for about 10-15 seconds. Add water, nutritional yeast, miso and salt and process on low speed again until well blended, about 1 minute. Scrape down the sides of the blender jar, then process on high speed until the mixture is completely smooth, for about 3-4 minutes.

Heat oil in a skillet on medium-high heat until shimmering. Add mushrooms and cook for about 7-10 minutes. Turn off the heat, stir in garlic and black pepper and cook using residual heat of the skillet until fragrant, for about 1 minute.

Meanwhile, cook pasta separately as per its packaging instructions. Drain and reserve 1/2 cup pasta water.

Transfer cooked pasta and sauce from the blender to the skillet with the mushrooms. Add lemon juice and toss until the sauce is thickened slightly and the pasta is well-coated, for about 1 minute. Adjust the sauce's consistency with pasta water if needed.

Top with chopped parsley and serve.