Preheat oven to 400 degrees. In a blender, puree tomatillos until smooth. In a large skillet, heat oil over medium-high. Add onion and jalapenos and cook until softened, 8 minutes. Add garlic and cook until fragrant, 30 seconds. Add tomatillo puree and bring to a boil. Reduce heat and simmer until sauce is thickened slightly, 10 minutes. Season with salt and pepper.
Briefly run stack of tortillas under cold water, turning to lightly dampen. Wrap stack in a clean kitchen towel and microwave until pliable, 30 seconds. Fill each tortilla with about 3 tablespoons chicken, fold in half, and arrange in a 9-by-13-inch baking dish, overlapping slightly. Top with tomatillo mixture, cover with foil, and bake until bubbling, 20 to 25 minutes. Let cool 5 minutes before serving.
In a small bowl, whisk together sour cream and 2 tablespoons water. To serve, drizzle casserole with sour cream mixture and sprinkle with cheese and cilantro.