Preheat oven to 400° F. 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. Grease a 9” by13” baking dish and then spoon about ½ cup of tomatillo enchilada sauce across the bottom. Arrange half of the tortillas in a baking dish, cutting to fit and overlapping as necessary. Top with half the chicken and half tomatillo sauce. Repeat layering with 6 tortillas, the rest of the chicken, and the remaining tomatillo sauce. Cover and bake until up to temperature, 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 cotija cheese and cilantro.