Heat the butter and oil in a 12-inch skillet over medium-high heat until the butter has melted. Add the leeks, garlic, 1/2 tsp. salt, and 1/4 tsp. pepper and cook, stirring often, until the leeks begin to brown, 3 to 5 minutes. Add the wine and simmer until reduced by half, about 1 minute. Add the chicken broth and bring to a simmer. Reduce the heat to low and press a 12-inch round piece of parchment over the leek mixture to cover completely (see the Test Kitchen blog for more on this technique). Cover the skillet and cook, stirring occasionally (you will have to lift the parchment), until the leeks are very soft but not falling apart and the garlic is very soft, about 40 minutes.
Meanwhile, bring a large pot of well-salted water to a boil and cook the spaghetti according to package directions until al dente. Drain, reserving 1/2 cup of the pasta cooking water.
Uncover the skillet and remove the parchment. Stir in the cream and cook, stirring occasionally, until the sauce thickens slightly, about 3 minutes. Add the cooked pasta and toss. If necessary, add the reserved pasta water 1 Tbs. at a time to make a silky sauce that clings to the pasta.
Off the heat, stir in the cheese and parsley. Season to taste with salt and pepper, and serve.
nutrition information (per serving):
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Photo: Scott Phillips