Combine the onions, red pepper flakes, pickling spice, salt, anise seeds, and bay leaf in a large heatproof bowl. Set aside.
In a large nonreactive saucepan, combine the vinegar and sugar and bring to a boil over medium-high heat, stirring to dissolve the sugar. Remove from the heat and pour over the onions, stirring the onions and spices with the liquid. Make sure the onions are submer