Gravy-Smothered Salisbury Steak

2 tablespoons unsalted butter

1/4 cup finely chopped onion

2 tablespoons all-purpose flour

2 cups beef broth

1 tablespoon Worcestershire sauce

1/2 teaspoon salt

1/4 teaspoon black pepper

1/2 teaspoon dried thyme

1/2 cup sliced mushrooms (optional)

Directions: