Preheat oven to 400 degrees.
To prepare enchilada filling, combine chicken and next 9 ingredients (chicken through black pepper) in a large bowl. Spread 1/2 cup salsa verde in bottom of a 13 by 9-inch baking dish coated with cooking spray. Warm tortillas according to package directions. Spoon about 1/3 cup chicken mixture down center of each tortilla; roll up. Arrange enchiladas, seam sides down, crosswise in dish. Pour remaining salsa verde evenly over enchiladas. Cover and bake at 400 degrees for 10 minutes or until thoroughly heated. Serve with sour cream, and garnish with sliced jalapeno, if desired.
To prepare salsa verde, discard husks and stems from tomatillos; cut into quarters. Combine tomatillos, 11/4 cups broth, 1/4 teaspoon salt, and chopped jalapeno in a saucepan over medium heat. Bring to a boil; reduce heat, and simmer 15 minutes or until tomatillos are tender. Cool slightly. Place salsa verde in a blender or food processor, and process until smooth or mash with a potato masher. Place a large nonstick skillet over medium-high heat until hot. Add salsa verde; cook until reduced to 2 cups (about 1 minute).