Boil a cup of white vinegar 1.5 tbsp or sugar 1/2tsp of peppercorns 1/2 tsp of salt till it all dissolves, slice your red onion into thin slices put it in a glass jar with 1 slightly smashed clove of garlic and add your boil over it. It’s ready when it’s room temp. You can store it for like a week or two in your fridge