Dice red onion and garlic cloves. Shred mozzarella cheese.
Thaw and drain spinach. Easy way is to set oven to 300 degrees and lay out frozen spinach or that still very moist cook it on low temp to remove moisture.
In a skillet on medium heat, sauté onion and garlic in butter (8-10 minutes cook time). Be sure to add in garlic half way though the cooking process and once you do reduce heat to medium and be sure to move around the onion and garlic so it doesn’t burn.
In a mixing bowl, combine sautéed onion and garlic, ½ cup of mozzarella, ½ cup parmesan, softened cream cheese, and milk (or cream). Mix well. Be sure mixture is cooled then fold in spin the spinach.
Heat skillet on medium high heat with oil. Season both sides of chicken with oil, salt, pepper, and garlic powder.
Once skillet is hot, sear each side of chicken for 2 minutes to make a crust. Let cool.
Cut slits into chicken breast for a cavity for spinach filling. Fill chicken breast with spinach mixture and top with more mozzarella cheese. Place stuffed breast back in skillet to prep for bake.
Bake at 350℉ for 12-15 minutes or until chicken reaches internal temp of 165℉.