Lightly coat an unheated medium saucepan with cooking spray. Add shallots; cook and stir over medium heat for about 6 minutes or until the shallots are tender and golden brown. Add mushrooms; cook for 4 minutes more. Stir together 1/2 cup of the broth and the flour; add to the saucepan. Add the remaining broth, the balsamic vinegar, and dried oregano (or marjoram), if using. Cook and stir until thick and bubbly; cook and stir for 1 minute more. Stir in fresh oregano (or marjoram), if using. Season with pepper.