Season the chicken all over with salt and pepper. Bring the chicken stock to a boil in a large saucepan. Place the breasts in the stock, reduce the heat to moderate, cover and cook until the meat is tender, about 15 minutes. Set aside to cool in the stock. When cool, remove the skin and bones and shred the meat into bite-sized pieces. Strain and reserve the stock for another use. In a large mixing bowl, combine the fresh tomatillo salsa, heavy cream, 1 teaspoon salt, 1/2 teaspoon pepper, the onion and shredded chicken pieces. Heat the vegetable oil in a medium skillet over medium-low heat. Cook the tortillas just about 5 seconds per side to soften and then transfer to a large colander to drain. See "Tomatillo Salsa" provided separately with this recipe. Preheat the oven to 350�F. Butter a 4-quart casserole. Combine the cheeses in a mixing bowl. To assemble the chilaquiles, spread a thin layer of the cheese mixture over the bottom of the baking dish. Push the solids in the bowl of chicken and tomatillo sauce to the side so the liquids form a pool in the bottom. Layer one third of the moist tortillas over the cheese and top with half of the chicken mixture with its sauce. Sprinkle half of the remaining cheese over the chicken. Repeat the layers, ending with a layer of torillas on top. Cover tightly with aluminum foil. Bake 30 minutes or until the edges are slightly brown. Let sit for 10 minutes before slicing and serving or unmolding. SOURCE: Mesa Mexicana, Mary Sue Milliken and Susan Feniger with Helena Siegel.