1. Cut a pocket into each chicken breast by placing one hand on top of the breast and inserting a knife into the thickest part. Cut an opening about 1/2 of the way through. Thoroughly dry the chicken on all sides with paper towels.
2. Combine cream cheese, mozzarella, spinach, onion, garlic and 1/2 tsp. of salt in a small bowl and mix.
3. Spoon the filling into the pocket of each chicken breast. Secure with a toothpick.
4. Heat oil over medium-high heat in a large skillet for about 3 minutes. Add chicken to the hot pan and cook for 5-7 minutes. Flip and cook another 5-7 minutes. Add butter to skillet and melt until chicken is golden brown on each side.
5. Remove chicken breasts from heat and let rest for 3 minutes before serving.
Serve with a warm slice of whole-wheat toast.