Step 1

Preheat oven to 350°. Toast walnuts on a rimmed baking sheet, tossing once, until golden brown, 6–8 minutes. Let cool, then finely chop. Bring a large pot of generously salted water to a boil.

Step 2

Vigorously mix walnuts, ricotta, lemon zest, garlic, oregano, and 2 oz. Parmesan in a large bowl until well combined. Add oil 1 Tbsp. or so at a time, stirring well after each addition, until all the oil is incorporated. Season pesto with salt and pepper.

Step 3

Cook pasta in pot of boiling water, stirring occasionally, until al dente. Scoop 1 cup pasta cooking liquid into bowl with pesto and stir gently to combine (it’s okay if some of the pesto is still in small clumps). Using a spider, tongs, or slotted spoon, transfer pasta directly to bowl with pesto. Mix energetically until pesto thickens enough to lightly coat pasta, about 2 minutes; the pasta should have sauce clinging to it but there should also be some extra sauce that collects in bottom of bowl.

Step 4

Divide pasta among bowls and serve with more Parmesan alongside for sprinkling over.