1. In a skillet, heat half of the olive oil. Saute onion, garlic and spinach for about 7 minutes, until onion is tender.
2. Rinse chicken breasts with cold water, pat dry with paper towels and butterfly (carefully slice each breast in half widthwise, but not all the way through, keep the other edge intact). Open the breast along the "fold" and pound lightly with a meat mallet (I usually cover them with plastic wrap for an easier cleanup).
3. Spread half of the spinach mixture on one half of the pounded breast.
4. Place mozzarella on top. Close pockets and secure with wooden toothpicks.
5. In a skillet, heat the rest of the oil and butter. Brown the chicken on both sides, cover, reduce heat and cook for an additional 15 minutes, or until cooked through.