Step 1

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.

Step 2

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.

Step 3

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.