Thinly slice shallots and place in a mixing bowl or clean canning jar.
Sprinkle peppercorns and {optional} red pepper flakes over the shallots.
In a saucepan, bring water, vinegar(s), salt, and sugar to a boil. Once it has boiled and the sugar has dissolved, remove from heat.
Slowly pour the liquid over the shallots. Stir. Note: the brine may not cover all the shallots at first.
Cover the bowl or canning jar and keep it at room temperature for 12 hours and then transfer to the fridge.
Notes
Optional seasonings - bay leaf, coriander, or mustard seeds.Will keep in the fridge for 2-3 weeks.You can sub in your favorite vinegar if you don't have apple cider vinegar on hand. Unseasoned rice vinegar, red wine vinegar, or using only white vinegar will all work.