Very finely slice red onions, either by hand or with a mandolin if you have one. Place onion slices into a sterilized mason jar with a lid.
3 medium-sized red onions
In a heatproof measuring jug, pour apple cider vinegar and boiling water. Stir in sugar and salt until dissolved.
1 1/2 cups boiling water, 1/2 cup apple cider vinegar, 1 tsp sugar, 1 tsp salt
Pour vinegar mixture into jar to cover sliced red onions. Put on lid and thoroughly shake the jar then allow to settle. Ensure the onions are submerged under the liquid- if they are not, push them down with a spoon. Seal jar with lid.
Place onion jar in the fridge and allow to marinate for at least 30 minutes before using. Pickled onions will last 3-4 weeks in the fridge.