Grind the cumin, peppercorns and sea salt together using a mortar and pestle, until you have a coarse powder.
Pour the water into a shallow dish. Gradually add the flour, shaking it through a sieve and beating with a fork. Stir in the ground spices and salt mixture.
Separate the onion slices into rings. Spread them out on a tray and lightly dust with flour, shaking off any exces