Step 1: Pat beef cubes dry with paper towels and season generously with salt and pepper.
Step 2: Heat olive oil in a large Dutch oven or oven-safe pot over medium-high heat. Brown the beef in batches until a deep crust forms on all sides. Remove beef and set aside.
Step 3: Add sliced onions to the pot, reducing heat to medium-low. Cook, stirring occasionally, for 30-40 minutes until deeply caramelized and dark brown. If they stick, add a splash of water or broth to deglaze.
Step 4: Add minced garlic and cook for 1 minute until fragrant.
Step 5: Stir in flour and cook for 2 minutes, stirring constantly, to create a roux.
Step 6: Deglaze the pot with red wine, scraping up any browned bits from the bottom. Bring to a simmer and cook for 5 minutes until reduced by half.
Step 7: Return beef to the pot. Stir in beef broth, fresh thyme, bay leaf, and Worcestershire sauce. Bring to a boil, then reduce heat to low, cover, and simmer for 2.5-3 hours, or until beef is fork-tender.
Step 8: While the stew simmers, prepare the cheesy bread. Preheat oven to 375°F (190°C). In a small bowl, combine melted butter and minced garlic. Brush one side of each bread slice with the garlic butter.
Step 9: Arrange bread slices on a baking sheet, buttered side up. Bake for 8-10 minutes, until lightly golden. Remove from oven.
Step 10: Once the stew is tender, remove the bay leaf. Taste and adjust seasoning with salt and pepper if needed.
Step 11: Turn on the broiler. Place the toasted bread slices directly on top of the stew, covering the surface. Sprinkle generously with Gruyere and Provolone cheeses.
Step 12: Broil for 3-5 minutes, watching carefully, until the cheese is melted, bubbly, and golden brown. Serve immediately.