Preheat the oven to 375F.
Cut the tart crust into small pieces and roll out individual tartlet shells.
Place the shells in a well-greased min cupcake tray, fill with baking weights and bake until crisp (about 30-45 minutes).
Meanwhile, toss the chicken and onion with the buffalo sauce in a bowl.
Remove the tartlets from the oven and fill with the chicken mixture, then top with bleu cheese.
Return the tartlet to the oven and bake until cheese is melted (about 10 minutes).