1 cup white wine
1⁄2 cup plus 3 tbsp. sherry
10 tbsp. butter
1 tsp. sugar
3 large yellow onions, thinly sliced Kosher salt and black pepper, to taste
6 sprigs flat-leaf parsley
6 sprigs thyme
2 fresh bay leaves
2 qts. Beef Stock
12!!1⁄2″-thick slices baguette
2 cloves garlic, smashed
6 cups grated gruyère cheese
2 cups finely grated parmigiano-reggiano
1. Heat oven to 425°. Combine wine, 1⁄2 cup of the sherry, 8 tbsp. of the butter, sugar, onions, and salt and pepper in a 9″ ! 13″ casserole dish and braise, uncovered, stirring occasionally, until the onions just begin to brown, 40–45 minutes. Remove casserole from oven, cover with foil, and continue braising in oven, stirring occasionally, until caramelized, about 1 hour more. Keep the onions warm.
2. Meanwhile, tie parsley, thyme, and bay leaves together with kitchen twine to make a bouquet garni. Put bouquet garni and stock into a pot and bring to a boil. Reduce heat to medium-low and simmer, partially covered, for 30 minutes. Remove and discard bouquet garni. Stir in remaining sherry and cook for 5 minutes more.
3. While the broth simmers, spread the baguette slices with the remaining butter. Toast in a skillet over medium heat, turning once, until golden, 5–7 minutes. Rub the slices generously with garlic and set aside. Discard any remaining garlic.
4. Heat broiler with rack 6″ from element. Arrange 6 heatproof bowls on a foil-lined sheet tray, divide onions and broth between bowls, and stir together. Place 2 baguette slices in each bowl; top each with about 1 cup gruyère and about 1⁄3 cup parmigiano. Broil until cheeses are browned and bubbly, 3–5 minutes. Serve immediately.