Start by peeling and thinly slicing the red onion.

In a medium saucepan, combine the white vinegar, water, sugar, salt, black peppercorns, and coriander seeds.

Place the saucepan over medium heat and stir until the sugar and salt dissolve.

Add the sliced onions to the saucepan and stir to coat them in the brine.

Let the mixture simmer for about 10 minutes, stirring occasionally.

Once the onions have simmered, remove the saucepan from heat and let it cool for a few minutes.

Carefully transfer the onions and brine to a jar.

Screw on the lid and refrigerate for at least an hour before serving.