Braised turnip greens with turnips and apples

Ingredients

2 pounds turnip greens or other braising greens (kale, collards, etc.), tough stems discarded and greens torn into small pieces.
Different greens can be added together.
1 (3/4-to 1-pound) ham hock, rinsed
3 1/2 cups water
3 Gala apples
1 1/4 pounds turnips, peeled and cut into 1/2-inch pieces
2 teaspoons cider vinegar
2 teaspoons sugar
2 tablespoons unsalted butter, cut into pieces

Directions

Bring greens, ham hock, water, and 1/4 teaspoon salt to a boil in a large heavy pot. Reduce heat to low and simmer, covered, stirring occasionally, until greens are almost tender, about 20 minutes. Meanwhile, peel apples and cut into 1/2-inch pieces. Add turnips and apples to greens with vinegar, sugar, and 1/2 teaspoon each of salt and pepper and cook at a bare simmer, covered, stirring and turning ham hock occasionally, until turnips and apples are tender but not falling apart, about 20 minutes more. Remove from heat and strain the liquid from the pot into another saucepan. Boil the liquid to reduce it to 1/3 or 1/4 of its original volume, then pour it back into the original pot with the greens. Stir in butter and salt to taste. Remove ham hock and finely chop any tender meat, discarding skin, bone, and tough meat. Add chopped meat to pot.