In small bowl, mix together sriracha and mayonnaise until well combined. Set aside.
In another bowl, combine boiling water, rice vinegar and sugar, stirring until the sugar is dissolved. Add carrots and radishes and toss to coat. (Can keep like this for up to 2 days)
Arrange cucumber slices in an even layer over the sauced buns
Divide chicken thighs over buns and top with pickled vegetables and cilantro
Optional Add Ins: This is a great way to use up leftover chicken, steak or fish