Season with salt, pepper, Worcestershire sauce, and beef broth; simmer until thickened (5-10 minutes).
Butter one side of each bread slice. Assemble sandwiches with beef mixture and cheeses, then grill until golden and cheese melts (3-4 minutes per side).
Slice and serve warm, garnished with parsley if desired.
Notes
For extra flavor, add a dash of hot sauce or your favorite herbs.