In a saute pan, bring the chicken stock , salt, garlic, and black pepper to a boil. Add the chicken breasts, and simmer for 15 to 20 minutes. Leaving chicken in the liquid, allow to cool. When cool, remove the chicken. Bone and shred the chicken breasts and return to the liquid. Set aside. Preheat oven to 450 degrees F. Heat the oil in a skillet. Pass the tortillas, one at a time, quickly through the oil, about 10 seconds per side. Place tortillas between paper towels to absorb excess oil. To assemble, put about 1/12th of the shredded chicken on each tortilla, sprinkling 1 teaspoon of the simmering liquid on top. Spread one tablespoon of the Salsa Verde over the chicken. Roll the tortilla around the chicken and sauce and place in a greased ceramic or glass casserole. Continue with the rest of the tortillas and filling until the entire filling is used up. Pour 3/4 of the remaining salsa verde over the enchiladas. Sprinkle with the grated cheese. Bake for 5 minutes then, check the progress. Continue baking until sauce begins to bubble, which may mean an additional 5 minutes. Top with sour cream when serving. Serve remaining salsa verde at the table. Note: Salsa verde can be made in advance and refrigerated for 3 to 4 days. Warm the refrigerated sauce before assembling the enchiladas.
For the sauce, clean the thin dry skins off the tomatillos. Boil all the ingredients together until soft, about 10 minutes. Cool for 30 minutes then, puree in a food processor. This should yield about 3 cups; if sauce seems too thick, thin with chicken stock to pourable paste consistency. Set aside.