In medium saucepan, combine together 1/2 cup (125 mL) water, white vinegar, apple cider vinegar, sugar, onion, pickling spices, salt and chili flakes, if using. Bring to boil, stirring frequently, until sugar dissolves. Remove from heat.
Place eggs in 4-cup (1 L) jar with lid. Pour hot liquid over eggs into each jar. Top up with more water, if necessary, to ensure eggs are completely covered. Cover jar with lid.
Let cool on counter, and refrigerate for at least two days before enjoying.