Preheat your oven to 400 degrees. Place your two whole chicken breasts in a deep sauté pan and cover completely with the two boxes of chicken stock. Bring to a boil and then immediately turn heat to low/simmer. Poach for 15 minutes then remove from liquid to cool.
Using two forks, shred your chicken up and set aside. In a mixing bowl, combine all your spices (chili powder, salt, garlic powder, cumin, and cayenne). Add in shredded chicken and toss to coat.
Take one tortilla and fill with about 3 tbs. of chicken, 1 tbs. sliced onion and sprinkle with about 1 tsp. of cheese (or more). Roll and lay seam down on a baking sheet sprayed with Pam/cooking spray. Continue with the rest of your tortillas then bake for 15 minutes.
While the flautas are baking, mix your Greek yogurt with the juice from 1/2 lime. Sprinkle in chili powder to taste.
When flautas are finished cooking, remove from oven and let cool for a couple minutes. Drizzle the yogurt sauce over the warm flautas and sprinkle with chopped cilantro. Enjoy!