For the roasted Tomatillo Sauce, Preheat broiler to high. Remove husks from tomatillos and rinse to remove the stickiness. Line a baking sheet with foil.
Place tomatillos, chilies, chopped onion and garlic on baking sheet. Drizzle with olive oil and season with salt and pepper. Place under broiler for 7-10 minutes, turning the vegetables once, until they are lightly charred and starting to soften.
Remove from the oven and let cool slightly. Remove the stems from the tomatillos and serrano chilies and peel the garlic. Add the vegetables and cilantro to the blender and puree. If salsa seems too thick, add chicken stock or water to thin it a bit.
For the Chilaquiles, heat about a half inch of vegetable oil in a large sauté pan over medium heat. When the oil is hot, fry the tortillas in small batches until they are slightly golden and almost crisp. Drain on paper towels.
Wipe out the sauté pan and add in the tomatillo salsa. Heat over medium heat until simmering. Add the tortillas and stir to combine. Cook until they are soft but not mushy, about five minutes. Season with salt and pepper to taste.
Add in cooked, shredded chicken.
To serve, place chilaquiles in individual bowls and top with cheese, sour cream, avocado and cilantro.