Step 1: Melt the butter in a large Dutch oven or oven-safe pot over medium heat. Add the sliced onions and 1 teaspoon of salt. Stir well to coat the onions in butter.
Step 2: Reduce the heat to medium-low and cook the onions, stirring occasionally, for 30-40 minutes, until deeply golden brown, soft, and jammy.
Step 3: Once the onions are caramelized, sprinkle in the flour and cook for 1-2 minutes, stirring constantly, to create a roux.
Step 4: Gradually whisk in the beef broth and sherry (if using). Bring the mixture to a gentle simmer, stirring occasionally, until it thickens slightly, about 5-7 minutes. Season with black pepper and adjust salt to taste.
Step 5: Stir in the fresh thyme sprigs (or dried thyme). Taste and adjust seasonings one last time.
Step 6: Preheat your oven to 375°F (190°C). Arrange half of the bread cubes evenly over the bottom of the Dutch oven, pressing them gently into the onion mixture.
Step 7: Sprinkle half of the shredded Gruyère cheese over the bread layer.
Step 8: Top with the remaining bread cubes, ensuring they are mostly submerged in the broth.
Step 9: Sprinkle the remaining cheese generously over the top.
Step 10: Place the Dutch oven, uncovered, into the preheated oven. Bake for 25-30 minutes, or until the casserole is bubbling around the edges and the cheese on top is golden brown and melted.
Step 11: Carefully remove the casserole from the oven. Let it rest for 5-10 minutes before serving. Garnish with fresh parsley if desired.