French Onion Beef Stew

For the beef stew

• 1 ½ lbs chuck steak cubed, 1.4 kg
• 2 tsp sea salt coarse
• 1 tsp black pepper
• 2-3 tbsp olive oil , or as needed
• ½ cup dry Marsala divided, 120 ml
• 2 tbsp butter unsalted
• 5 onion large
• salt large pinch
• 1 tsp brown sugar
• 1 tbsp thyme fresh
• 4 garlic minced, cloves
• 1 tbsp Worcestershire sauce
• 1 tbsp Dijon mustard
• 1 tbsp tomato paste (optional)
• 1 tbsp flour
• 2½ cups beef broth stock, 620 ml
• 2 bay leaf

For the cheesy bread topping

• 1 white baguette , sliced
• olive oil to brush on the bread, as needed
• 2 tbsp Dijon mustard , or as needed
• 1 ½ cups gruyere cheese coarsely grated, 150g
• 3 tbsp Parmesan Cheese , freshly grated
• ½ tbsp thyme to garnish, fresh