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Combine vinegar, water, sugar, salt and yellow mustard seeds in a medium-sized pan and heat it slowly on low heat.
Meanwhile peel the red onions, divide them into thin rings and add them to the warm liquid. Turn the heat off and let it cool off. You can use them immediately, but you can also store them for about a week in the fridge.
If you want to store them longer you can also add them into sterile jars when they're still hot. These onions are great in a salad, on a sandwich or on a tortilla. Enjoy!