04Lay the pie crusts out flat on a clean surface and with a 2 1/2-inch round cutter cut out 12 circles from each crust. Press them into the cups of the mini-muffin tin. Fill each cup 1/2 way with the cooled onions. Pipe the cheese mixture on top of the onions, filling the cups. Bake the tarts until the filling is set, about 15 to 20 minutes. Let the tarts cool slightly, and then remove them from the muffin tin and serve.