Thinly slice the onions. Heat 2 tablespoons of olive oil in a Dutch oven over medium heat. Add onions, cooking for 20-25 minutes until caramelized.

Pat the chuck steak dry, season with salt and pepper. In medium-high heat, add 1 tablespoon of olive oil and sear the beef in batches for 4-5 minutes on each side.

Reduce heat back to medium, add 2 tablespoons of butter to remaining fat in the pot. Stir in caramelized onions with brown sugar and salt; cook for 10-15 minutes.

Add minced garlic and thyme to the onions; cook for 2 minutes. Pour in Marsala wine and simmer for 3-4 minutes.

Stir in Worcestershire sauce, Dijon mustard, and tomato paste. Add seared beef and 4 cups beef broth. Toss in bay leaves. Bring to a gentle boil.

Preheat oven to 320°F (160°C). Cover pot and transfer to oven; cook for 90 minutes or until beef is tender.

Slice the baguette, spread Dijon mustard on top of each slice, and stack on a baking tray. Sprinkle with Gruyere and Parmesan cheese.

Bake cheesy bread in the oven for the last 10-15 minutes until bubbly and golden.

Remove stew from the oven, discard bay leaves. Serve stew in bowls, top with cheesy bread, and broil for 2-3 minutes if desired.