Cut the onions into half rings and place them in a clean container (500ml) with a lid.
Heat up the water, apple cider vinegar, maple syrup and salt and (doesn’t have to boil) and pour over the onions. Let it cool down, put the lid on and put the jar in the fridge.
You can eat the onions very soon, but they will have their most beautiful color after about 24 hours. They should last for approximately 2 weeks.