How to use onion skin as fertilizer?

Boil a large handful of onion skins in 1 litre of water for 20 minutes. Cool, strain, and dilute 1:3 with plain water. Apply to pots monthly. Alternatively air-dry skins and crumble them directly into potting mix — they decompose slowly releasing potassium, calcium, and trace minerals. Avoid using charred or salted skins.