Slowly Caramelize the Onions: In a large heavy-bottomed Dutch oven, melt the butter with the olive oil over medium-low heat. Add all the sliced onions and toss to coat in the fat. Cook uncovered for 15 minutes, stirring occasionally, until soft and wilted. Sprinkle with sugar and 1/2 teaspoon of salt. Reduce heat to low and cook for another 35-40 minutes, stirring every few minutes, until the onions are deeply caramelized, jammy, and rich mahogany brown.

Build the Flavor Base: Stir in the minced garlic and cook for 1 minute until fragrant. Sprinkle the flour over the caramelized onions and stir constantly for 2 minutes to cook off raw starch.

Deglaze (Alcohol-Free): Pour in the apple cider vinegar and balsamic mixture (or white grape reduction), using a wooden spoon to scrape up all the flavorful browned fond stuck to the bottom of the Dutch oven.

Simmer the Rich Broth: Slowly pour in the beef bone broth while stirring. Add the Worcestershire sauce, fresh thyme sprigs, bay leaves, remaining salt, and black pepper. Bring to a gentle boil, then reduce heat to low and simmer uncovered for 25-30 minutes to allow the herbal flavors and deep onion sweetness to meld completely. Remove and discard the thyme sprigs and bay leaves.

Toast the Baguette Rounds: While the soup simmers, arrange the baguette slices on a baking sheet. Brush lightly with olive oil and bake at 400°F (200°C) for 6-8 minutes until golden and thoroughly dried out like croutons.

Broil to Bubbly Perfection: Position your oven rack 6 inches below the broiler and turn the broiler to HIGH. Place oven-safe soup crocks or bowls on a sturdy rimmed baking sheet. Ladle hot soup into each crock, leaving 1/2 inch of space at the top. Float 1 or 2 toasted baguette slices on top of each bowl. Generously drape each bowl with shredded Gruyère cheese, making sure the cheese covers the bread and reaches the crock edges. Broil for 3-5 minutes until the cheese is blistered, bubbly, and melted golden brown. Let rest 3 minutes before serving hot!