We love a rich, silky, aromatic cream sauce, and this is one of our favorite excuses to make one. (When it’s luxuriously enveloping a boatload of greenery, it’s good for you, see?) It’s an utterly indulgent, festive, wintry side dish — and if you want to make it a meal, we can’t imagine anyone objecting to tossing it with spaghetti or pappardelle (just add a few splashes of the pasta cooking water to thin out the cream sauce).
In a large sauté pan with a cover, warm the butter and olive oil over medium-high heat. Add the leeks and garlic cloves, season with salt, and sauté for 5 minutes, stirring regularly, until the leeks are fully softened (if they look like they might start to brown, turn down the heat). Add the chile flakes, lemon juice and zest and loads of black pepper, stir well, and add the cream.
Let the cream come to a simmer (you want some gentle bubbles here) and then add as much kale as the pan can hold in a single layer. Cover the pan, lower the heat to medium, let the kale wilt for a minute or so, then give it a thorough stir to coat it with the cream. Add a splash of water (2-3 tablespoons) and another sprinkle of salt; cover again, and let cook for another minute or two, stirring it occasionally, until the kale is wilted enough to make room for more.
Repeat as many times as needed (but don’t add water unless your cream has cooked away too quickly) until all the kale is fully wilted in the cream. Lower the heat even more and let it cook a bit longer. You want it to cook way down, until it’s dark, soft, and creamy. Before serving, check for seasoning: You may want a bit more black pepper, a bit more salt and a final sprinkle of chile flakes.