PREHEAT oven to 400ºF
Place chicken breast between two sheets of plastic wrap. Using a meat mallet, thin the breast down to about 1/2" thickness. [mv_img id="4662"]
Using a mandolin or a knife, slice the potatoes, thinly, about 1/4" thick. Place two large sheets of aluminum foil on the counter. Spray foil with cooking spray if not using nonstick foil. Evenly divide the sliced potatoes between the two sheets, fanning the potatoes in the center of the sheet. [mv_img id="4663"]
Remove top sheet of plastic wrap from chicken and spread 1/2 Tablespoon of taco seasoning mix on each chicken breast. Place chicken on top of potatoes, then apply 1/2 Tablespoon to other side of chicken.
Top each beast evenly with salsa and both kinds of cheese. [mv_img id="4664"]
Bring up both long lengths of foil. Double fold top and both ends to seal each packet, leaving room for heat circulation inside. Place both packets on a rimmed baking sheet. [mv_img id="4665"]
Bake for 35 minutes or until chicken has reached 165º. Let stand 5 minutes before opening packets.
Garnish with sour cream and scallions.