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Slice the top and bottom off the onion and then slice onion in half, from top to bottom. Remove the outer peel. Lay the cut surfaces of the onion onto the cutting board, and then slice into half moons about 1/8" thick using a very sharp knife. I also recommend using a mandoline for slicing the onions, especially if you like super thin or very even slices.
In a medium saucepan, whisk together apple cider vinegar, red wine vinegar, sugar, and salt. Place over medium-high heat and bring mixture to a boil. Whisk until sugar and salt dissolve, then remove pan from heat and whisk in allspice and red pepper flakes.
Carefully add sliced onion to the pan and gently stir to combine. Let mixture cool completely at room temperature, stirring occasionally.
Pour into a glass container (I like to use a large canning jar), cover tightly with a lid, and refrigerate until thoroughly chilled. The pickled flavor will intensify over time, but it's good to go after resting overnight in the refrigerator. This recipe is for refrigerator pickling only and needs to be stored in a refrigerator. Enjoy for up to one month.