Melt butter in a Dutch oven over medium heat. Add onions and 1/2 tsp salt, cover, and soften for 5 min.

Uncover and caramelize onions very slowly over medium-low heat, stirring occasionally, until golden brown (45–60 min).

Meanwhile, warm beef stock in a saucepan over low heat.

Add Chardonnay and sherry to onions, bring to a boil. Stir in flour and cook for 1–2 min.

Gradually add warm stock, 1/4 tsp salt, and pepper. Boil uncovered for 10 min. Adjust seasoning.

Heat broiler. Place ovenproof bowls on a baking sheet, ladle in soup, top with baguette slices, and sprinkle generously with fresh shredded Gruyère.

Broil 1–2 min until cheese melts and browns. Serve immediately.