1. Place walnuts in a food processor; process until finely ground. Set aside.
2. Sprinkle chicken with 1 teaspoon salt and 1/4 teaspoon pepper. Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add half of chicken to pan; cook 4 minutes on each side. Remove from pan. Repeat procedure with remaining chicken.
3. Recoat pan with cooking spray. Add onion and garlic to pan; sauté 3 minutes. Add remaining 3/4 teaspoon salt, remaining 1/4 teaspoon pepper, ground walnuts, broth, and the remaining ingredients except cilantro, stirring with a whisk until smooth. Bring to a boil; reduce heat, and simmer until reduced to 1 1/2 cups (about 15 minutes), stirring occasionally. Return chicken to pan; cover and simmer 10 minutes or until thoroughly heated. Garnish with cilantro sprigs, if desired.