Add butter to pan with the drippings. (Use same pan burgers were cooked in)
Saute mushrooms and onions until golden brown (5 minutes)
Add beef broth and Worcestershire sauce, bring to a boil and reduce heat. Cook for 2 to 3 mins.
In a separate bowl, mix cornstarch with 1 tablespoon water to make a paste. Slowly add into gravy, stirring until thickened.
Season with salt and pepper. Cover and keep warm until ready for use.