Trim the top and bottom of the onion. Slice the onion in half, from top to bottom. Remove the outer peel. Lay the cut surfaces of the onion onto the cutting board. Slice into half moons about 1/8" thick using a very sharp knife. If preferred, a mandoline works well for slicing the onions, especially if you like super thin slices.

In a medium saucepan, whisk together the apple cider vinegar, red wine vinegar, sugar and salt. Place the pan over medium-high heat and bring the mixture to a boil. Whisk until the sugar and salt fully dissolve. Remove the pan from the heat and whisk in the allspice and red pepper flakes.

Carefully add the sliced onion to the pan. Gently stir to combine. Allow the mixture to cool completely at room temperature, stirring occasionally. Pour the entire mixture into a glass container. Cover tightly with a lid and refrigerate until thoroughly chilled. Canning jars work well here, either one large or two small.

The pickled flavor will intensify over time, but they're ready to enjoy after resting overnight in the refrigerator.

Your pickled onions will be good for up to four weeks if kept in the fridge. During that time, be sure that the onion slices are fully submerged in the pickling liquid.