Melt your butter in a hot frying pan. Sauté the spinach and onion in the butter. Set the stove to medium-high heat and leave the vegetables to cook for about 3 minutes until soft.
Mix the cream cheese in the pan. Allow dissolving for around 2 minutes. Stir to combine with the onion and spinach. Set aside.
Slice a pocket in the chicken breast, deep enough to be filled. Rub both sides of the chicken with salt and pepper. Season all side, including the inside of the pocket.
Jampack the pocket with spinach filling and some shredded cheese. Close the breast and secure with a toothpick.
Heat the olive oil in a frying pan over medium heat. Gently place the chicken on the oil and cook for 7-10 minutes with the lid on. Turn the chicken over and let it fry for another 7-10 minutes. Remove from the heat once golden.
Cut in the middle to spill the filling. Enjoy while warm.