Step 1

Preheat oven to 350°. Toast cashews on a rimmed baking sheet, tossing halfway through, until golden brown, 7–9 minutes. Let cool.

Step 2

Heat 2 Tbsp. oil in a large Dutch oven or other heavy pot over medium-high. Arrange half of mushrooms in a single layer in pot and cook, undisturbed, until edges are brown and starting to crisp, about 3 minutes. Give mushrooms a good toss, then continue to cook, tossing occasionally, until all sides are brown and crisp, about 5 minutes more. Using a slotted spoon, transfer mushrooms to a plate; season with salt. Repeat with remaining 2 Tbsp. oil, remaining mushrooms, and more salt.

Step 3

Reduce heat to medium-low and return all of the mushrooms to same pot. Add leeks, shallots, and garlic and cook, stirring often and adding another 1 Tbsp. oil if pan looks dry, until leeks and shallots are translucent and softened, about 4 minutes. Remove from heat.

Step 4

Meanwhile, cook pasta in a large pot of boiling salted water, stirring occasionally, until very al dente, about  2 minutes less than package directions (the pasta will finish cooking in the pan). Drain pasta, reserving ¾ cup pasta cooking liquid.

Step 5

Blend cashews, nutritional yeast, and pasta cooking liquid in a blender until very creamy. Set cashew cream aside.

Step 6

Return drained pasta to pot with mushroom mixture and add reserved cashew cream. Place over medium heat and cook, stirring well to coat, until pasta is al dente, about 3 minutes. Remove from heat. Add parsley and squeeze in juice from lemon half. Toss well to incorporate.

Step 7

Divide pasta evenly among shallow bowls or plates and top with several grinds of pepper.