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1
Melt the butter in a wide, heavy-bottomed pot over medium heat. Add all the sliced onions and the pinch of sugar. Stir to coat. Cook at medium heat, stirring every 5 minutes, for 45–60 minutes. The onions are ready when they have collapsed to a fraction of their original volume and have turned deep amber to mahogany — the colour of a well-tanned complexion, darker. Any pale golden colour means more time is required. Any black spots mean the batch is compromised; they cannot be corrected and will introduce bitterness throughout. This is where the dish lives or dies.
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2
Add the beef stock to the caramelised onions and bring to a simmer, scraping up any residue from the base of the pot. Simmer for 15–20 minutes until the stock has absorbed the full depth of the onion flavour and the liquid has a body drawn from the stock's collagen. Taste for seasoning.
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3
Ladle the soup into oven-safe bowls. Float two slices of toasted baguette on the surface of each bowl and cover generously with grated Gruyère. Place under a hot broiler until the cheese melts, bubbles, and achieves deep brown spots across its surface.