1. Melt the butter in a saucepan.
2. Add the washed fish bones, sliced onion, lemon juice and herbs.
3. Cover with greaseproof paper and a lid and sweat for 5 minutes without coloration in order to extract the juices from the bones.
4. Cover with cold water, bring to the boil and skim any impurities that rise to the surface, then simmer for 20 minutes.
5. Strain into a clean pan, reboil and use as required.