Penne with Broccoli Rabe, Walnuts, and Pecorino

• 1/4 cup olive oil
• 2 1/2 cups chopped leeks (white and pale green parts only; about 2 large
• 2 large bunches broccoli rabe (rapini),* stemmed, coarsely chopped
• 4 garlic cloves, peeled
• 3/4 cup vegetable broth
• 1/3 cup creme fraiche or whipping cream
• 1 Tbsp finely grated lemon peel
• 1 cup grated Pecorino Romano cheese, divided
• 1 pound penne
• 1 1/2 cups walnut halves, toasted

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, creme 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.