Season the cubed beef with salt and pepper, then sear in heated oil until browned and set aside.
In the same pot, melt butter and add sliced onions with salt and sugar; cook until caramelized, about 25-30 minutes.
Stir in garlic, thyme, and half the Marsala wine to deglaze the pan. Scrape up browned bits for flavor.
Return beef to pot, add remaining wine, beef broth, Worcestershire sauce, mustard, tomato paste, flour, and bay leaves. Simmer or pressure cook until beef is tender.
For cheesy bread: brush baguette slices with olive oil, toast, then spread mustard and top with Gruyere and Parmesan.
Ladle stew into bowls, top with cheesy bread slices, and broil until cheese is melted and golden.
Garnish with thyme and reserved caramelized onions. Serve and enjoy!
Notes
Stew tastes even better the next day. Store in fridge or freeze for longer storage. Cheesy bread optional but highly recommended!