Boil the chicken breasts in water until thoroughly cooked, and then shred them. Meanwhile, slice the mushrooms, chop the green onions, and mince the garlic. In a large bowl, combine the soups, milk, and salsa with a whire whisk. Add the mushrooms, green onions, garlic, oregano, and pepper, and stir to combine. Set aside. Cut the tortillas into quarters. Wash the chiles and remove the seeds. I usually run my finger along one side of the chile to open it up, and then I just wash the seeds away with the running water. Dry them on paper towels. Grate the cheese. Preheat the oven to 350. In a large buttered casserole dish, layer 1/2 of the tortillas, the shredded chicken, the remaining tortillas, the whole chiles, the sauce, and the grated cheese. Bake at 350 for 40 to 45 minutes or until the cheese is lightly browned.