Position a rack in the upper third of the oven and preheat to 375 degrees . In a medium skillet, heat 1 tbsp. oil over medium heat. Add the onion, garlic and jalapeno; cook until the vegetables are soft, about 5 minutes. Add the cumin and cook for 1 minute. Add the tomatillos; season with salt and pepper. Cook for 2 minutes, then stir in 1/2 cup water and simmer until the tomatillos are soft, about 5 minutes.
Let the mixture cool, then transfer to a blender and mix until smooth, about 1 minute. Add 3/4 cup sour cream; blend until combined. Add the cilantro and blend until smooth; season.
Spread 1/2 cup of the salsa into a 9-by-13-inch baking dish. In a bowl, mix the chicken with 1 cup of the salsa.
In a small skillet, heat the remaining 7 tbsp. oil over medium-high heat. Working with 1 tortilla at a time, submerge in the oil for a few seconds, then transfer to paper towels to drain, keeping the tortillas in a single layer.
Divide the chicken mixture among the tortillas, spooning about 1/3 cup in a line down the center of each. Roll them up and line them in the prepared baking dish. Pour the remaining 1 cup of salsa over the enchiladas.
Cover the dish with foil and bake until hot and golden, 20 to 25 minutes. Uncover, sprinkle with the cheese and bake until the cheese is melted and the salsa is bubbling, about 5 minutes longer. Serve with the lettuce and more sour cream.