2 Tbs. unsalted butter
1 Tbs. extra-virgin olive oil
6 medium leeks (light-green and white parts only), halved and sliced crosswise into 1/2-inch pieces (10 cups)
10 medium cloves garlic, halved
Kosher salt and freshly ground black pepper
1/2 cup dry white wine
1 cup lower-salt chicken broth
12 oz. dried spaghetti
1/2 cup heavy cream
1/2 oz. grated Pecorino Romano (1/3 cup)
1/4 cup chopped fresh flat-leaf parsley
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.
Cover the skillet and cook, stirring occasionally, 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 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.
Add the cream and cook, stirring occasionally, until the sauce thickens slightly, about 3 minutes.
Add the cooked pasta and toss. Off the heat, stir in the cheese and parsley. Season to taste with salt and pepper, and serve.