In a large Dutch oven or soup pot, melt the butter over medium heat. Add the oil, onions, salt, pepper, and sugar. Cook, uncovered, stirring occasionally, until the onions are deeply golden brown and caramelized, about 45 to 55 minutes. Stir more frequently as they brown, scraping the fond from the bottom of the pan. Reduce heat or add water if necessary to prevent burning.

Pour in the wine, increasing the heat to high. Cook, stirring continuously, until the liquid has nearly evaporated and the onions have a jam-like consistency, approximately 8 to 10 minutes.

Sprinkle in the flour and cook, stirring continuously, for one minute.

Add the beef broth, Worcestershire sauce, thyme, and bay leaves. Bring the mixture to a boil, then lower the heat and simmer, covered, for about 30 minutes.

While the soup simmers, preheat the oven to 400°F (204°C). Place the baguette slices on a baking sheet in a single layer and bake until they are dry, crisp, and golden at the edges, around 10 minutes.

Remove the bay leaves from the soup. Add the sherry, and adjust seasoning as needed. Enhance flavor with additional Worcestershire sauce or sugar if necessary.

Position an oven rack 6 inches from the broiler and preheat the broiler. Place broiler-safe crocks on a baking sheet, fill each with hot soup, and top with 1 or 2 baguette slices. Sprinkle Gruyère and Parmigiano Reggiano cheeses evenly over the top. Broil until the cheese is melted and bubbling around the edges, 3 to 5 minutes. Allow to cool slightly before serving.

For regular soup bowls, place cheese-topped baguette slices under the broiler to melt the cheese, about 2 minutes, then add to each soup bowl.

To store, refrigerate the soup (without bread or cheese) for up to 3 days or freeze for up to 3 months. Prepared toasts can be kept at room temperature for up to 3 days.