Gruyère and Cider Fondue

1 cup hard apple or pear cider
1 tablespoon cider vinegar
4 cups (1 pound) shredded Gruyère
1 tablespoon plus 2 teaspoons cornstarch
1 tablespoon Calvados, applejack, or Poire William
Freshly ground black pepper
Cubes of crusty bread, cored and sliced Granny Smith apples or Bosc pears, and sliced grilled kielbasa, for serving

Many people serve fondue as a meal, but it is also a lovely appetizer to serve with a light-bodied red wine.

Combine the cider and vinegar in a nonreactive medium saucepan. Stirring to dissipate the bubbles, bring to a simmer over medium heat. Reduce the heat to medium-low; the liquid should barely simmer.
Toss the Gruyère with the cornstarch in a large bowl to coat the cheese. A handful at a time, whisk the cheese into the simmering cider mixture, whisking until the first batch is melted before adding more. When all the cheese has been added, return the heat to medium and heat just until the fondue bubbles a few times; do not overcook. Remove from the heat and stir in the Calvados. Season with pepper.
Transfer to a fondue pot set on its trivet over the flame. Arrange the bread, apples, and sausage for dipping on a platter. Serve the fondue hot with fondue forks and the platter of dipping ingredients.