Prepare the Onions: Slice 3 red onions into rings or half-rings according to your preference.
Mix the Ingredients: In a hermetically sealed container, add the sliced onions, coriander seeds, Jamaican pepper, black peppercorns, and thyme.
Prepare the Pickling Solution: In a saucepan, combine 20cl of cider vinegar, 20cl of water, 50g of sugar, and 1 tablespoon of pink salt. Heat over medium until the sugar and salt dissolve.
Pickle the Onions: Pour the boiling mixture over the onions. Ensure the onions are fully submerged. Seal the container and let it cool at room temperature. Refrigerate and keep for several weeks.