Slice the onions by cutting the top end off, keeping the root intact. Peel and halve the onions without slicing all the way through. Thinly slice using a mandoline or sharp knife.
Season the cubed beef with salt and pepper. Heat olive oil in a large casserole dish and sear the beef in batches until browned. Set aside.
Deglaze the pan with ¼ cup (60ml) of Marsala, scraping off the browned bits. Simmer until the Marsala reduces.
Preheat the oven to 375°F (190°C). Brush the baguette slices with olive oil and toast in the oven for 15 minutes until golden.
Caramelize the onions by cooking them with butter, salt, and sugar until browned. Add garlic, thyme, and the rest of the Marsala. Cook until caramelized.
Reduce the oven temperature to 320°F (160°C). Stir in Worcestershire sauce, mustard, tomato paste, flour, and beef broth. Simmer, then add the beef and bay leaves. Cover and cook in the oven for 90 minutes.
Adjust seasoning of the stew and discard bay leaves.
Spread Dijon mustard on the toasted bread, top with Gruyere and Parmesan. Divide the stew into ovenproof bowls, cover with bread slices.
Add extra cheese on top and broil for a few minutes until melted and browned. Watch closely to prevent burning.
Garnish with thyme and reserved caramelized onions before serving.
Notes
For a quicker version, you can use store-bought French onion soup instead of caramelizing the onions from scratch.
Keyword Beef Stew with Cheesy Bread Topping Recipe