Cheese Tortellini with Walnut-Parsley Pesto

Place walnuts, oil, parsley, 3 tablespoons Parmesan cheese, garlic, salt, and pepper in a food processor or blender; pulse into a coarse puree.

Fill a large pot with lightly salted water and bring to a rolling boil; stir in tortellini and return to a boil. Cook uncovered, stirring occasionally, until the tortellini float to the top and the filling is hot, 7 to 8 minutes. Drain tortellini, reserving 1/2 cup cooking water.

Toss tortellini with 1/4 cup of the reserved pasta water, walnut pesto, and butter. Add more of the cooking water if mixture seems dry. Sprinkle with remaining 1 tablespoon Parmesan cheese and pass more at the table.