Place chicken in a single layer in a large nonstick skillet and cover with water. Add cilantro sprigs and 1 clove of garlic. Cook chicken breasts over medium heat until no longer pink in center, about 10 to 15 minutes. Remove chicken from pan and let cool; reserve liquid.
Shred cooled chicken and set aside. Pour reserved cooking liquid into a bowl and add ancho peppers. Cover and soak peppers about 10 minutes.
Heat a small skillet over medium heat. Add remaining garlic cloves and roast in pan, turning frequently, until blackened in spots. Remove to a small bowl and when cool enough to handle, peel garlic.
Place peppers, garlic, tomato sauce and cumin in jar of a blender and blend on high until smooth, about 3 to 4 minutes. Pour mixture into a saucepan, add chicken and heat through.
To make polenta, bring 2 1/2 cups of water to a boil; add salt. Slowly add cornmeal, stirring until smooth, about 6 to 8 minutes.
Coat a 9 x 9-inch pan with cooking spray and pour polenta into pan. Smooth with a spatula. Allow polenta to firm up, about 1 to 2 minutes. Cut into 4 slices and remove from pan. Place polenta slices in a large nonstick skillet coated with cooking spray. Cook on both sides until crispy, about 2 to 4 minutes per side.
To serve, place one slice of polenta on a plate and top with 3/4 cup of chicken mixture. Garnish with cilantro, radishes and scallions.