Soupe à l'oignon gratinée — the French onion soup with a bread and Gruyère gratin topping — requires onions cooked for 45–60 minutes to a deep brown caramelisation before any broth is added. The onion caramelisation is the entire flavour development — the fructose and glucose in the onions undergo Maillard reactions and caramelise, producing the 300+ aromatic compounds that make the soup's flavour. An onion soup made in 20 minutes tastes of sweet onion broth; one made in 60 minutes tastes of something entirely different.
- **The onion quantity:** More than seems necessary — the onions reduce to a fraction of their raw volume. A 4-person soup requires 1.5–2kg of raw onions. - **The caramelisation time:** 45–60 minutes at medium heat, stirring every 5 minutes. The colour target: deep amber to mahogany — the colour of a well-tanned complexion, deeper. - **No rushing:** Onions caramelised over high heat burn at the edges before the centres are soft — producing bitter spots throughout. - **The sugar:** A pinch added in the first 10 minutes accelerates the Maillard reactions without adding sweetness. - **The broth:** Beef stock — its collagen adds body to the thin onion preparation. - **The gratin:** Toasted baguette slices floating on the soup's surface, covered with grated Gruyère — gratinéed under a broiler until the cheese bubbles and achieves deep brown spots. Decisive moment: The colour of the onions at 45 minutes. Deep amber: the soup will be good. Pale golden: under-caramelised, needs more time. Any black spots: burned — these patches cannot be corrected and will add bitterness throughout.
France: The Cookbook