Making a pocket in the chicken breast to hold the stuffing is easy with a good, sharp, thin-bladed knife. Browning the chicken in a skillet before baking gives it a beautiful golden color, and finishing in the oven ensures that it cooks evenly throughout.
- Preheat oven to 400°F. Use a baking sheet with sides and lightly coat it with cooking spray.
- Mix cheese, ham, mustard and pepper in a small bowl.
- Cut a horizontal slit along the thin, long edge of a chicken breast half, nearly through to the opposite side. Open up the breast and place one-fourth of the filling in the center. Close the breast over the filling, pressing the edges firmly together to seal. Repeat with the remaining chicken breasts and filling.
- Lightly beat egg white with a fork in a medium bowl. Place breadcrumbs in a shallow glass dish. Hold each chicken breast half together and dip in egg white, then dredge in breadcrumbs. (Discard leftovers.)
- Heat oil in a large nonstick skillet over medium-high heat. Add chicken breasts; cook until browned on one side, about 2 minutes. Place the chicken, browned-side up, on the prepared baking sheet. Bake until the chicken is no longer pink in the center or until an instant-read thermometer registers 170°F, about 20 minutes.