1 pound bag chopped kale OR 1 1/2 pounds kale
4 tablespoons (1/2 stick) butter
1/2 small white onion, finely chopped
6 cloves garlic, minced
1/4 cup all-purpose flour
2 cups whole milk
1/3 cup finely grated Pecorino or Parmesan cheese
1/2 teaspoon dry mustard
1 teaspoon smoked paprika
1 teaspoon salt, or to taste
Freshly ground black pepper
If using whole kale leaves, remove the stalks and major ribs and discard. Chop roughly.
Set a large bowl of ice cold water in the sink. Bring a large pot of salted water to a boil. Add the chopped kale and simmer for 1 minute, then drain into a large strainer and immediately transfer to the bowl of ice water. Swish the kale around in the cold water, then drain and squeeze dry with paper towels or a kitchen towel.
Melt the butter in a deep sauté pan or 3-quart saucepan over medium-high heat. Add the onion and garlic, and cook, stirring constantly, for 1 minute. Sprinkle the flour over the onion mixture and cook, stirring, for 3 minutes. Turn the heat down if necessary; do not let the flour brown.
Whisk in the milk. Cook the mixture, stirring slowly and continuously, until the sauce comes to a boil and thickens enough to coat the back of the spatula, about 5 minutes. Reduce the heat to low and continue cooking until the sauce is the consistency of soft pudding, about 2 more minutes.