Boil at least 4 quarts of salted water. Cook the pasta according to the package directions, drain, and keep warm.

Meanwhile, in a large skillet, heat the butter and olive oil over medium heat. When the butter is melted, add the leeks to the pan. Saute, stirring often until the leeks are softened and just starting to brown.

Add the garlic and saute another minute or two.

Add the heavy cream and reduce heat to medium low. Cook the cream until reduced by about half. Once the cream is reduced, add the lemon zest, rosemary, and cheese. Serve warm.