Step 1

Cut leeks in half lengthwise; rinse thoroughly under water. Cut crosswise into thin slices, place in a colander and rinse again.

Step 2

Heat 2 teaspoons butter in a 12-inch cast-iron skillet over low heat, add pancetta (or bacon) and cook, stirring, until almost crisp. Transfer to a paper towel-lined plate with a slotted spoon.

Step 3

Add the remaining 1 teaspoon butter and the leeks to the pan. Season with 1/8 teaspoon each salt and pepper and add 1/4 cup broth (or water); simmer, covered, until tender, about 5 minutes.

Step 4

Add cabbage and the remaining 1/4 cup broth (or water); season with the remaining 1/8 teaspoon each salt and pepper. Cover and simmer until tender, about 15 minutes. (Check the cabbage after 5 minutes; if necessary, add more broth or water, a tablespoon at a time, to prevent scorching.) Stir in mushrooms; cover and continue simmering until the vegetables are tender, 5 to 7 minutes more. Serve sprinkled with the reserved pancetta (or bacon).