Mix together dry ingredients (flour, cornflour, flax seed, salt, pepper and baking powder), and to these mix in the water to form a smooth batter. Allow to stand for minimum 20 minutes to thicken, then stir once more before use.
Peel the onions and cut them in to about 1 - 1.5 cm slices horizontal to their top and root, producing slices you can separate in to rings. Use the good sized rings for this recipe, the small pieces from each onion's center can be used in another recipe.
Heat a pan of oil (need at least 6 cm depth) to 160 °C (320 °F)
Dip your onion slices (or whatever else you’re coating, eg. pepper slices, mushrooms, etc.) in the batter with a fork, tap off the excess batter and then immediately, and carefully transfer to the hot oil. Add each ring so that it is not touching another ring, so they do not stick together. Be careful to avoid the oil splashing.
Fry until golden brown and crisp, flipping over once halfway through to cook both sides. Drain on absorbent paper.