Part 1: Caramelizing the Onions (The Most Important Step)

1. Melt Fat: In a large, heavy-bottomed pot or Dutch oven, melt the butter and olive oil over medium-low heat. The combination of fats prevents the butter from burning too quickly while adding rich flavor.
2. Add Onions: Add the thinly sliced onions to the pot. It will look like an enormous amount, but they will cook down significantly. Stir to coat them evenly in the butter and oil.
3. Slow Cook: Cook the onions, stirring occasionally (every 10-15 minutes), for 45-60 minutes. This is a slow and low process. Do not rush it by turning up the heat, as the onions will burn instead of caramelizing.
4. Develop Flavor: After about 20 minutes, the onions will soften and become translucent. After 40 minutes, they will start to turn a beautiful golden-brown. At this point, sprinkle them with the 1 teaspoon of sugar and 1 teaspoon of salt. The sugar helps with browning, and the salt helps draw out more moisture.
5. Finish Caramelizing: Continue cooking and stirring more frequently for another 15-20 minutes, until the onions are a deep, rich brown color and have a jam-like consistency. They should be incredibly sweet and fragrant. Once done, remove the onions from the pot and set them aside in a bowl.

Part 2: Preparing the Beef Filling

1. Brown the Beef: Increase the heat under the same Dutch oven to medium-high. Add the ground beef to the pot. Use a wooden spoon to break it apart. Cook until the beef is thoroughly browned and cooked through, about 8-10 minutes. Drain off any excess grease from the pot.
2. Add Aromatics: Add the minced garlic and freshly ground black pepper to the beef. Cook for another minute until the garlic is fragrant, being careful not to let it burn.
3. Create the Roux: Sprinkle the 1/4 cup of all-purpose flour over the beef. Stir continuously for 2 minutes. This will cook out the raw flour taste and create a roux that will thicken the sauce beautifully.

Part 3: Building the Sauce and Assembling the Casserole

1. Deglaze the Pan: If using, pour the red wine into the pot. Scrape the bottom of the pot with your wooden spoon to release all the browned bits (the “fond”). This is where a huge amount of flavor lives. Let the wine bubble and reduce by about half, which should take 2-3 minutes.
2. Create the Gravy: Slowly whisk in the beef broth, Worcestershire sauce, and soy sauce. Continue to stir until the sauce is smooth and begins to thicken. Bring the mixture to a gentle simmer.
3. Combine Ingredients: Return the caramelized onions to the pot. Add the fresh or dried thyme. Stir everything together until the beef and onions are fully coated in the luscious gravy. Let it simmer for 5 minutes for the flavors to meld. Taste and adjust seasoning with more salt or pepper if needed.
4. Prepare the Baking Dish: Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch (or equivalent 3-quart) casserole dish.
5. Assemble: Pour the beef and onion mixture into the prepared casserole dish and spread it into an even layer.

Part 4: Baking to Perfection

1. Add Topping: Arrange the cubed bread evenly over the top of the beef mixture. Gently press them down slightly so they absorb some of the sauce.
2. Add Cheese: In a bowl, toss the grated Gruyère and provolone/mozzarella cheese together. Sprinkle the cheese mixture evenly over the bread cubes, ensuring you cover everything right to the edges.
3. Bake: Place the casserole dish on a baking sheet to catch any potential drips. Bake, uncovered, for 25-30 minutes, or until the sauce is bubbly and the cheese on top is completely melted, golden brown, and slightly crispy in spots.
4. Rest and Serve: Remove the casserole from the oven and let it rest for at least 10 minutes before serving. This allows the sauce to set up slightly and makes it easier to serve. Garnish with fresh chopped parsley for a pop of color before bringing it to the table.