Cooking to melting
The difference between an onion that is soft and an onion that has melted, and why the second one is the only one this book uses.
Most recipes tell you to soften an onion for five minutes until translucent. That is not what this book means. Cooked to melting is a further stage altogether, where the structure of the vegetable has broken down so completely that there is nothing left to catch. An onion cooked to melting has no strands. A leek cooked to melting has no threads. That distinction is the whole reason onion and leek appear here at all.
Why it matters
Strands are what a narrowed bowel cannot manage. The fibre in an allium runs lengthways in long parallel bundles, and a five-minute sweat leaves those bundles intact and slippery.
How to do it
Cut small and cut evenly before you start, because a thick slice will still be a thick slice at the end. Fat, low heat and time do the work. Melt butter or oil in a wide pan, add the vegetable with a pinch of salt to draw water out, cover for the first ten minutes, then uncover and let it go. Twenty to thirty minutes is normal. Stir often enough that nothing colours, because browning firms the edges rather than softening them. If the pan dries, add a splash of water rather than turning the heat up.
The cue
Press a piece against the side of the pan with the back of a spoon. It should collapse into a smear with no resistance and no squeak. There is a sound to under-cooked leek, a faint rubbery note, and once you have heard it you will not miss it again. The heap in the pan will have lost half its volume and turned from white to a soft ivory.
Common mistakes
Turning the heat up to save time. Adding acid or salt-heavy liquid early, which sets the pieces. Stopping at translucent. And skipping the step that follows in most of these recipes, which is sieving the blended sauce so that anything which did not melt never reaches the plate.