Step 1

Place cashews in a large bowl and pour in cold water to cover by 2"; let soak overnight. Drain cashews; set aside. (If you have a high-speed blender, feel free to skip the soaking step.)

Step 2

Heat 2 Tbsp. oil in a large high-sided skillet until shimmering. Cook mushrooms, stirring often, until browned, about 5 minutes; season with salt. Transfer mushrooms to a small bowl and set aside; reserve skillet.

Step 3

Place reserved cashews in a blender. Add garlic, miso, and ½ cup fresh water. Blend on high speed until smooth, adding a little more water if needed to get mixture moving.

Step 4

Cook bucatini in a large pot of generously salted boiling water until very al dente, about 2 minutes less than package directions (pasta will finish cooking in sauce).

Step 5

Meanwhile, heat remaining 2 Tbsp. oil in reserved skillet over medium. Add red pepper flakes, onion powder, and 2 tsp. black pepper. Cook, stirring, until fragrant, about 1 minute. Pour in cashew purée and stir vigorously to combine (it’s okay if the mixture seizes up). Scoop out 1½ cups pasta cooking liquid and add to skillet; whisk until sauce is creamy, adding more pasta cooking liquid as needed to achieve a smooth, silky sauce. Stir in lemon juice; taste and season with more salt if needed.

Step 6

Using tongs, transfer pasta directly to sauce in skillet. Add most of reserved mushrooms to skillet. Cook (still over medium), tossing often, until sauce is hot and pasta is al dente, about 2 minutes.

Step 7

Top with lemon zest, parmesan, and more black pepper; scatter remaining reserved mushrooms over.