½ cup walnuts
½ cup fresh ricotta
Zest of 1 lemon
1 garlic clove, finely grated
2 tsp. finely chopped oregano
2 oz. Parmesan, finely grated, plus more for serving
¼ cup extra-virgin olive oil
Freshly ground black pepper
1 lb. tube- or rod-shaped pasta (such as rigatoni or spaghetti)
Preheat oven to 350°F. 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.
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.
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.
Divide pasta among bowls and serve with more Parmesan alongside for sprinkling over.