Combine the goat cheese and egg white in a small bowl.
Butterfly each chicken breast and divide the filling between the breasts. Close the flap around the filling. Sprinkle the outside with salt and pepper.
Heat the butter and oil in a large oven-safe skillet over medium-high heat. Sear both sides of the chicken.
Transfer to the oven and bake for an additional 15-20 minutes or until 160 on a meat thermometer.
Optional: Add the tomatoes and onions to the pan when transferring to the oven. When the chicken is done, add the baby spinach to the pan under the chicken. It will wilt quickly.
Notes
Nutrition: These are large chicken breasts. This meal could easily serve 6 or even 8 people if you slice them and serve them with generous portions of sides.