Place tomatillos, onion, 3 tablespoons pepitas, 2 tablespoons cilantro, garlic, broth and 1/4 teaspoon salt in a blender or food processor; process until smooth. Transfer to a medium saucepan and cook over medium-high heat, stirring occasionally, until reduced to about 3/4 cup, 12 to 15 minutes. The mixture will resemble a thick paste. Remove from the heat and stir in sour cream. Set aside.

Meanwhile, place one piece of chicken at a time between sheets of plastic wrap and pound with a meat mallet or heavy skillet until flattened to an even 1/4-inch thickness.

Whisk egg white and water in a shallow dish until combined. Whisk cornmeal and the remaining 1/4 teaspoon salt in another shallow dish. Dip each chicken breast in egg white, then dredge in cornmeal, turning to coat evenly.

Heat 1 1/2 teaspoons oil in a large nonstick skillet over medium heat. Cook 2 pieces of the chicken until browned on the outside and no longer pink in the middle, 2 to 4 minutes per side. Transfer to a plate and tent with foil to keep warm. Wipe out the pan, reduce the heat slightly and repeat with the remaining oil and chicken. Serve the chicken with the reserved sauce and garnish with the remaining 1 tablespoon pepitas and 1 tablespoon cilantro.

Hulled pumpkin seeds, also known as pepitas, are medium-dark green and have a delicate nutty flavor. They are sold salted, roasted and raw, and can be found in the bulk-foods section of natural-foods stores or Mexican groceries.