In a medium saucepan, combine broth, apple cider, onion, maple syrup, cider vinegar, and bay leaves. Bring to boiling; reduce heat. Simmer, covered, for 15 minutes or until onion is very soft. Remove the bay leaves; discard.
In a large saucepan, melt butter over medium heat. Stir in the flour and pepper. Carefully add hot cider mixture all at once. Cook and stir over medium heat until thickened and bubbly. Add sage. Cook and stir 1 minute more.
