Mix spinach, Parmesan cheese, cream cheese, 3/4 of the French-fried onions, lemon juice, garlic, sea salt, and black pepper together in a bowl; spread into a glass baking dish.
Bake in preheated oven until heated through and thick, about 15 minutes. Sprinkle remaining French-fried onions over the top of the spinach mixture and continue baking until the onions are brown, about 5 minutes.
Heat coconut oil in a large skillet over medium-high heat. Gently lie chicken into the hot coconut oil, place a lid on the skillet, and cook until no longer pink in the center and the juices run clear, 5 to 7 minutes per side. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).
Put chicken onto plates and top with generous amounts of the spinach mixture.