In a small mixing dish combine the chili powder, cayenne pepper, and salt & pepper. Rub this seasoning all over the chicken breasts, then transfer to a small, ungreased skillet.
In another mixing bowl combine the chicken broth and lime juice, whisking well. Pour this mixture over the chicken breasts until they are nearly submerged, then poach, covered, over a medium flame for 15-20 minutes until each breast reaches an internal temperature of 165F. Once cooked through, transfer chicken to a large cutting board to rest for several minutes.
Pre-heat oven to 350F. Pour several tablespoons of the salsa verde into the bottom of a 9″ baking dish, spreading evenly.
Use two forks to shred each chicken breast apart. Next, build your enchiladas with a layer of shredded chicken, sliced onion, and green chile in the center of each tortilla, then wrap tightly and place seam side down into the baking dish. Repeat until all tortillas have been rolled, then top with the remaining salsa verde, green chiles, and pepper jack cheese. Bake for 15 – 20 minutes until the cheese has melted, then immediately plate and serve!