Very thinly slice the red onions from root to stem, place them in a glass jug and cover with boiling water.

Let sit for one minute, then drain and place in a mason jar.

In a small pot; heat the water, vinegar and salt to just under a boil and pour over the onions. Make sure the onions are fully submerged.

Put on the lid and let cool to room temp; place in the fridge overnight. Add to sandwiches, salads, curries and soups.