1. Preheat oven to 375°F on bake setting.

2. In large skillet over medium-high heat, warm olive oil. Add onions to skillet and sauté until translucent, about 3-4 minutes.

3. Add spinach to skillet, cover, reduce heat to medium and cook until spinach is wilted. Transfer onion and spinach to a large bowl and allow to cool for 3-4 minutes.

4. Add dill, Feta cheese, Monterey Jack cheese, salt and pepper to bowl; mix well to combine.

5. Sprinkle chicken with salt and pepper.

6. Place eggs in small bowl and panko in pie plate. Spray baking sheet with cooking spray and set aside.

7. Using sharp knife, slit each chicken breast along long side, cutting only about 3/4 of the way through. 

8. Divide spinach mixture into four sections and stuff each breast with a portion of the mixture. 

9. Dip breasts first in egg mixture and then in panko to coat.  Place chicken on prepared pan and lightly coat chicken with olive oil cooking spray. 

10. Place in preheated oven and cook until the spinach mixture and the internal temperature of the thickest part of chicken reaches 165°F. Always use a meat thermometer to determine the temperature.