Place chicken in a bowl and pour in enough Italian salad dressing to coat. Marinate in the refrigerator, 2 to 3 hours.
Preheat oven to 375 degrees F (190 degrees C).
Drain and discard marinade from chicken and season chicken with garlic powder and lemon pepper. Transfer seasoned chicken to a casserole dish. Layer spinach over chicken.
Mix cream cheese, chives, olive oil, garlic, salt, and pepper together in a bowl. Slowly pour chicken broth into cream cheese mixture, stirring until mixture is smooth; pour over chicken and spinach. Cover casserole dish with aluminum foil.
Bake in the preheated oven for 20 minutes. Remove foil and top chicken mixture with mozzarella cheese; cook until chicken is no longer pink in the center and cheese is melted, 15 to 20 minutes.
2 cups cubed boneless chicken breast
1 cup Italian salad dressing, or as needed
1/2 teaspoon garlic powder
1/4 teaspoon lemon pepper
1 (8 ounce) package fresh spinach
2 (8 ounce) packages cream cheese, softened
1/2 cup chopped fresh chives
1/4 cup olive oil
1 clove ga