How to Make It
Preheat the oven to 350°. Bring a large pot of salted water to a boil. Spread the walnuts on a baking sheet and toast for 8 to 10 minutes, until they are fragrant and lightly browned. Let cool completely. Coarsely chop half of the walnuts and set the remaining toasted walnuts aside.
In a food processor, pulse the remaining walnuts with the basil and garlic until the walnuts are finely chopped. With the machine on, add the olive oil in a thin stream and process until the pesto is almost smooth. Add 1/2 cup of the cheese and pulse until just incorporated. Transfer the walnut pesto to a bowl and season it with salt and pepper.
Add the broken lasagna noodles to the boiling water and cook until al dente. Drain the pasta, reserving 1/2 cup of the pasta cooking water. Return the drained pasta to the pot. Add the watercress and walnut pesto and toss well. Add the reserved pasta water and toss again until well coated. Transfer the pasta to bowls, garnish with the remaining 1/4 cup of cheese, the chopped walnuts and sautéed mushrooms and serve.
One Serving: 525 cal, 27 gm fat, 4.7 gm sat fat, 64 gm carb, 2 gm fiber.