To prepare salsa verde, discard husks and stems from tomatillos; cut into quarters. Combine tomatillos, 1 1/4 cups broth, 1/4 teaspoon salt, and chopped jalapeño 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).
Preheat oven to 400°.
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 x 9-inch baking dish coated with cooking spray. Warm tortillas according to package directions. Spoon about 1/3 cup chicken mixture downcenter 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° for 10 minutes or until thoroughly heated. Serve with sour cream, and garnish with sliced jalapeño, if desired.