Two roast chicken carcasses and four hours of near-silence make two litres of the most useful thing in a kitchen.
Snap the carcasses into pieces small enough to pack tightly into the pot. This is not cosmetic: a tightly packed pot needs less water to cover, and less water means a more concentrated stock without any extra reduction later.
Cover by about three centimetres with cold water. Cold, not hot — starting cold draws soluble protein out gradually and it coagulates into a scum you can lift off. Starting hot seizes it into the liquid and it stays there as cloudiness.
Bring it to a bare simmer over about forty-five minutes. There is no hurry and hurrying costs you clarity.
A grey foam will rise as it comes up to temperature. Take it off with a spoon and discard it. After the first twenty minutes there will be almost none, and after the first hour there will be none at all.
Not at the start. Four hours reduces vegetables to nothing and their flavour goes flat and slightly sweet. An hour to ninety minutes in the pot is all they want.
That is the target: a bubble breaking the surface every three or four seconds, nowhere near a simmer. Lid off or barely ajar.
This is the step that separates good stock from bad, and it requires nothing but leaving it alone and glancing at it every half hour.
Through a fine sieve into a clean pot or bowl. Let it drip. Pressing the solids forces fine particles through and turns a clear stock cloudy in one careless movement.
Stand the container in a sink of cold water and stir occasionally. Getting it through the warm range quickly matters for keeping.
Overnight in the fridge the fat sets into a solid cap on top. Lift it off and keep it — that is schmaltz, and it is the best fat in any kitchen for roasting potatoes. Underneath, a good stock will have set to a soft jelly.
Stock is an ingredient, not a dish. It will go on to be reduced in a pan sauce, boiled down for a braise, or used as the base of a soup that is seasoned at the end. Salt added now is concentrated by every one of those steps, and by the time you notice it is far too late.
The same applies to strong herbs. Rosemary and sage take over. Bay, peppercorns and parsley stalks are the whole list.