In a small bowl, combine the cheese, 4 tablespoons olive oil and the lemon peel.
In a large pot of boiling, salted water, cook the pasta until al dente. Drain, reserving 1/2 cup of the cooking water; return the pasta to the pot.
Meanwhile, in a large skillet, heat the remaining 2 tablespoons olive oil over medium-high heat. Add the garlic and crushed red pepper and cook, stirring, for 1 minute. Add the escarole and cook, stirring frequently, until wilted but still crisp; season with salt. Stir the escarole and the reserved pasta cooking water into the pasta. Transfer to a platter; dollop with the cheese mixture and sprinkle with the walnuts. Toss lightly just before serving.