Slice and caramelize onions with butter, brown sugar, and thyme.
Brown beef cubes on all sides for deep flavor.
Add mustard, Worcestershire, flour, and beef broth; stir well.
Cover and bake until tender and thickened.
Toast slices, spread Dijon, add cheeses.
Top stew with cheesy toasts; broil until golden.
Sprinkle thyme and serve warm.
