Heat the oven to 375 degrees.

Place all the thawed spinach is a colander. Squeeze as much water out of the spinach as you can using your hands or the back of a large spoon pressing against the sides of the colander.

Thoroughly mix the spinach, ricotta cheese, egg, salt and pepper in a mixing bowl.

Spray a 9 X 9 baking pan with cooking spray. Line the sides and bottom of the pan with half of the muenster cheese.

Scoop the spinach mixture into the cheese lined baking pan and smooth the top to make it flat. Place muenster cheese on top until covered.

Bake uncovered for 30 minutes. Let cool slightly for 5 minutes before serving