Bring a large pot of water to a boil. Add a generous pinch of salt and cook tortellini just to al dente, according to package directions. Before draining the tortellini, scoop about 1/2 cup of the cooking water out into a liquid measuring cup.
8-10 ounces cheese tortellini
While the tortellini cooks, toast the walnuts, if using, in a dry skillet set over medium heat for 2-3 minutes, just until they are fragrant and develop a few brown spots. Remove walnuts and set aside, leaving the skillet over medium heat.
1/4 cup chopped walnuts
Add pesto to the skillet, then slowly stir in the cream. Warm for 3-4 minutes, until smooth. Add pasta water, a small splash at a time, until the sauce is thinned out to your desired consistency. Stir in the lemon juice, followed by the drained tortellini. Stir well to combine.
1/2 cup basil pesto, 1/3 cup heavy cream, 1 teaspoon lemon juice
Sample and season with salt and pepper to taste. (You may not need any, depending on how salty your pesto was to begin with. I usually don’t add any.)
Serve tortellini right away, with toasted walnuts, fresh-grated Parmesan, and fresh basil, if desired, on top.
Parmesan and fresh basil