1 pound tart, crisp apples (3 to 4 apples, such as Granny Smith, Jonagold, or Winesap, peeled, cored, an cut into 1-ich chunks (about 3 cups) 1 large yellow onion, thinly sliced (about 2 cups) 1 teaspoon finely chopped fresh rosemary 2 tablespoons extra-virgin olive oil 1 tablespoon unsalted butter, melted Kosher salt and freshly ground black pepper 2 cups fresh apple cider 1 tablespoon pure maple