Gather all ingredients and cube the chuck steak. Slice the onions thinly for even caramelization.
In a heavy-bottomed pot, heat olive oil over medium-high heat. Season beef with salt and pepper; sear until browned on all sides. Remove from pot.
Add butter and onions to the pot with a pinch of salt and brown sugar; cook over medium-low heat for 20–25 minutes until caramelized.
Stir in garlic and thyme; deglaze with half of the grape juice.
Return beef to pot along with remaining grape juice, Worcestershire sauce, Dijon mustard, flour, broth, and bay leaves; stir well.
Bring to a boil then reduce heat to low; simmer uncovered for about 90 minutes until beef is tender.
Preheat broiler; brush baguette slices with olive oil and broil until golden brown.
Serve stew hot in bowls topped with melted cheese on toasted baguette slices.
