Pour vinegar, water and salt into a small saucepan and heat on high until it just starts to boil. Stir until the salt is completely dissolved then remove from heat. Allow to cool until it reaches room temperature. If you are short on time, heat only a little bit of the water to dissolve the salt, then add the rest of the water and the vinegar to the hot mixture to cool it down.

Measure the red pepper flakes and black peppercorns into the bottom of a clean jar.

Remove the stem and roots from the radishes then slice them as thinly as you can with a knife or mandolin. Slice the onion in thin slices next and put the radishes and the onions in the jar on top of the spices. 

Once the brine has cooled, pour it into the jar so that the radishes and onions are completely covered. 

Place a plastic or glass lid on the jar. If you are using a regular metal jar lid, lay a sheet of parchment paper over the top of the jar before placing and securing the lid.

Place the jar in the fridge and leave to chill at least 4-6 hours before enjoying. Store in the fridge for a couple months after that.