Step 1

Bring a large pot of salted water to a boil. Meanwhile, pulse the walnuts in a food processor until finely chopped; remove to a small bowl. Add half the mushrooms to the food processor; pulse until finely chopped. Slice the remaining mushrooms.

Step 2

Heat the olive oil in a large dutch oven over medium-high heat. Add the walnuts; cook, stirring, until toasted, about 2 minutes. Add the onion and chopped mushrooms and cook, stirring, until the mushrooms are dry, 5 minutes. Season with salt and pepper and cook until the mushrooms are browned, 2 to 3 minutes. Add the sliced mushrooms and garlic and cook until softened, 3 to 5 minutes. Add the tomato paste and cook, stirring, until coated, about 1 minute. Add the crushed tomatoes, then rinse the empty can with 1/2 cup water and pour it into the pot. Add the rosemary and bring to a simmer. Cook, stirring, until the sauce thickens slightly, about 10 minutes.

Step 3

When the sauce is almost ready, add the pasta to the boiling water and cook as the label directs for al dente, about 5 minutes. Using tongs, transfer the pasta to the sauce. Cook, tossing, until well coated. Stir in half the parsley and season with salt and pepper; remove the rosemary sprig. Divide the pasta among bowls and top with the remaining parsley.