Preheat oven to 400 degrees F (205 degrees C).
Into a mixing bowl, add Frozen Spinach (1 cup), Ricotta Cheese (1/2 cup), and Egg (1). Mix well, mashing the cheese to make the mixture as smooth as possible.
Add Onion (1), Salt (1/2 teaspoon), and Ground Black Pepper (1/2 teaspoon). Mix well.
Into a casserole pan, add Butter (2 tablespoon). Lay two halves of the Chicken Breast (2) on top.
Spread half of the spinach mixture over the chicken. Place the other two pieces of chicken on top. Season with Salt (1/2 teaspoon), Ground Black Pepper (1/2 teaspoon), Dried Rosemary (to taste), and Dried Thyme (to taste).
Cover with foil. Bake for 20 minutes.
Remove the foil. Bake for an additional 10 minutes.
Cover again with the foil. Let rest for 10 minutes.
Cut in half and serve alone or with sides.