6 tablespoons unsalted butter
1/2 cup sugar
1 1/4 teaspoons coarse salt
2 medium Granny Smith or other tart, firm apples (1 pound total), peeled, cored, and cut into 1 1/2-inch wedges
1 large onion, peeled and cut into 1 1/2-inch wedges
1 small (1 1/2 pounds) red cabbage, cored and cut into 1 1/2-inch wedges through the core
3/4 cup apple cider vinegar
3/4 cup water