Step 1

In a food processor or blender, pulse the walnuts, parsley, garlic, Parmesan, oil, salt, and pepper to a coarse puree.

Step 2

In a large pot of boiling, salted water, cook the tortellini until just done, about 4 minutes for fresh and 12 minutes for frozen. Reserve 1/2 cup of the pasta water. Drain the tortellini. Toss with 1/4 cup of the reserved pasta water, the walnut pesto, and the butter. If the pasta seems dry, add more of the reserved pasta water. Sprinkle with additional Parmesan and pass more at the table.