Penne with Broccoli Rabe, Walnuts, and Pecorino

Heat oil in heavy large skillet over medium-high heat. Add leeks and sauté until golden and soft, about 5 minutes. Add broccoli rabe and sauté, tossing with tongs, until slightly softened and bright green, about 3 minutes. Add garlic and stir 1 minute. Stir in broth, crme frache, and lemon peel, then 1/3 cup cheese. Season to taste with salt and pepper. Meanwhile, cook pasta in large pot of boiling salted water until just tender but still firm to bite, stirring occasionally. Drain, reserving 1 cup pasta cooking liquid. Return pasta to pot. Stir in broccoli rabe mixture and walnuts, adding 1/2 cup cooking liquid to moisten and adding more by tablespoonfuls if needed. Divide pasta among bowls. Serve, passing remaining cheese separately. An Italian leafy green vegetable with scattered clusters of tiny, broccoli-like florets; available at some supermarkets and at specialty foods stores.