In a large saucepan bring broth to a boil; add chicken breasts, garlic cloves and peppercorns. With cover on bring to a boil then reduce heat.
Simmer until chicken is thoroughly cooked and will shred easily (approximately 1 hour).
Remove chicken from broth and shred with forks.
Combine shredded chicken with 1 cup of salsa, mushrooms, roasted red peppers, chilies, scallions, and cilantro.
Put 1/4 cup of enchilada sauce on the bottom of a casserole dish.
Remove tortillas from oven. Increase heat to 350 degrees F.
Put some filling into each of the tortillas and roll. Place filled tortillas in casserole dish seam side down. Combine remaining salsa and enchilada sauce and spoon over enchiladas, top with cheese and bake uncovered for 30 minutes or until cheese is brown and bubbling.