Prepare Onions: Cut the red onions into rounds or half-rounds, according to preference.
Add Spices to Container: In a container that can be sealed tightly, add allspice, coriander seeds, black peppercorns, and thyme slices.
Add Onions: Place the cut onions into the container with the spices.
Prepare Vinegar Brine: In a saucepan, mix water, cider vinegar, cane sugar, and salt. Bring the mixture to a boil.
Combine and Seal: Pour the boiling vinegar brine over the onions in the container. Seal the container tightly.
Cool and Store: Let the container cool to room temperature, then store it in the fridge. The pickles can be kept for weeks or even months, improving in taste over time.