López-Alt's testing of caramelised onion technique produced one of his most cited findings: properly caramelised onions take 45–60 minutes minimum, not the 10–15 minutes claimed by most recipes. The shortcuts — baking soda, high heat, added sugar — produce different results that are faster but not equivalent. Understanding what is actually happening during the long slow cook explains why shortcuts fail.
Onions cooked in fat over medium-low heat for 45–60 minutes until deeply browned, sweet, and reduced to approximately one-fifth of their original volume. The transformation involves water evaporation (the onions lose 75–80% of their weight as moisture), sugar development, and Maillard browning — all of which require time.
Properly caramelised onions provide a sweet-savoury depth that raw, sautéed, or quickly browned onions cannot replicate. They are the foundation of French onion soup, the base of a great burger topping, the soul of a tarte aux oignons. The time investment cannot be bypassed without producing a categorically different ingredient.
- Start with more onions than seem necessary — they reduce dramatically. 1kg raw onions produces approximately 200g caramelised [VERIFY ratio] - Medium-low heat — high heat browns the exterior before the interior has softened and the water has evaporated, producing burnt edges and raw centres - Salt added at the start draws out moisture faster, accelerating the first stage [VERIFY effect] - Deglaze with liquid (water, wine, stock) when the fond builds on the pan base — this fond is concentrated flavour; scraping it up and incorporating it is essential to the final depth - Baking soda accelerates browning by raising pH (Maillard reaction proceeds faster at higher pH) but produces a different, slightly soapy flavour at higher concentrations [VERIFY: small amount — approximately 1/4 tsp per kg] - The onions are done when they are uniformly deep brown, smell of caramel and savoury depth, and have reduced to a fraction of original volume Decisive moment: The colour at 30 minutes — the onions should be golden and significantly reduced but not yet at their final depth. If they are already dark at 30 minutes, the heat is too high and the exterior is burning before the interior is fully cooked. If they are still pale and barely reduced, the heat needs to increase slightly.
- Claiming they're done at 15 minutes — they are golden at best, not caramelised - Heat too high — burning the exterior while the interior remains harsh - Not deglazing the fond — the most flavourful material in the pan is being discarded - Adding too much baking soda — soapy off-flavour
THE FOOD LAB (continued) + THE DUCHESS BAKE BOOK