Prepare Onions: Slice onions thinly and set aside.
Sear Beef: Season beef and brown in olive oil until caramelized.
Caramelize Onions: Cook onions with butter, brown sugar, and thyme until golden.
Build Stew: Add mustard, Worcestershire, tomato paste, flour, and beef broth. Return beef and simmer.
Slow Cook: Bake covered at 320°F (160°C) for 1½ hours.
Prepare Bread: Toast baguette slices brushed with olive oil, spread with Dijon, and top with cheeses.
Assemble: Place cheesy toasts over stew and broil until bubbly.
Serve: Garnish with thyme and enjoy immediately.
