Broil poblano peppers 5 minutes per side or until blackened. Place in a small paper bag; seal. Let stand 10 minutes; peel and chop. Bring chicken broth and tomatillos to a boil in a saucepan over medium heat. Cover and simmer 8 minutes. Remove from heat; let stand 20 minutes. Pour into a blender. Add lime juice and garlic cloves; process until smooth. Pour into a bowl; stir in poblanos, onion, cilantro, kosher salt, and chopped serrano chile. Chill.