Place about 1 inch of water in a 12" skillet, and add whole chicken breasts, roughly sliced onion, smashed garlic and hot peppers (if using).
Bring water to a boil over medium high heat, then reduce to simmer. Cover and cook chicken until instant read thermometer reaches 165 degrees at thickest part of breast, about 20 minutes.
Remove breast from water and strain and discard vegetables, saving chicken broth for sauce.
Enchiladas
Melt butter in small skillet over medium eat, and cook onion and pepper until just soft, about 5 minutes.
Shred chicken into large bowl. Add onion and pepper mixture, green chiles, cumin, green salsa, 6 ounces of cheese (half), and cilantro. Combine mixture thoroughly.
Spray large baking dish (ours is 9 x 13 and 12 enchiladas fit perfectly). Preheat oven to 350 degrees.
Place one tortilla on flat surface and place about 1/2 cup of the chicken mixture along side (towards middle). Roll up from filling side, and place, seam side down, in prepared baking dish. Continue with remaining filling. Sprinkle remaining sharp cheddar over enchiladas.
Enchiladas can be prepared to this point up to a day in advance: cover and chill until ready to bake.
Combine broth and heavy cream and pour over enchiladas. Cover pan with foil and bake for 30 minutes. Remove foil and bake another 15 minutes or so until brown and bubbly.