Slice the red onions thin and place in a large, deep bowl. You can use a knife, but a mandoline makes it faster and easier.
3 large Red Onions
Add the vinegar, water, spices, sugar, and salt to a sauce pan and heat to boiling, then simmer for 5 minutes.
1½ cup Vinegar, ¾ cup Water, ½ cup Sugar, 1 teaspoon Sea salt, 6 Allspice berries, 1 Dried Red Chile
Pour the hot pickling brine over the bowl of sliced red onions.
Cover the bowl loosely with some plastic wrap, letting it dip down to reach the brine surface.
Place a small saucer plate or canning weights (they look like clear round hockey pucks!) on top of the plastic wrap to submerge the onions into the brine completely.
Let the onions marinate in the pickling brine for an 30 minutes. You can also let them sit overnight.
Pack the onions into clean, sanitized jars up to 1" from the top.
Wipe the rims and place the sealing discs and screw caps on the jars.
Refrigerate the pink pickled onions until cold before serving. For the best flavor, chill for 24 hours.