Place the vinegar, water, salt and sugar in a small saucepan on the hob and bring to a simmer. Simmer for 30-60 seconds, stirring. Turn off the heat and let this cool completely.
Put the onions in a jar and pour over the liquid. Put in the fridge for at least 15 minutes.
Notes
We haven't added anything extra into ours in terms of flavour, but you could add peppercorns, pickles jalapeños, cumin seeds, fennel seeds garlic cloves or a bay leaf.
You can either use a really sharp knife, or a mandolin to slice the red onions. Try to get them as thin as possible.
These can be ready in as little as 15 minutes, but for the best pickled red onions, let them marinate for 24 hours.