Preheat oven to 350 degrees F (175 degrees C).
Cook the frozen spinach in the microwave per the package instructions, subtracting a minute from the total cook time. Squeeze off the liquid from the spinach either by hand, or by placing it in a colander & pressing it down with a spoon or your fingers.
In an 2 quart baking dish, combine all your ingredients - reserving ¼ cup of mozzarella cheese.
Bake in the preheated oven 30 minutes, or until cheeses are melted and bubbly. Remove, & sprinkle on remaining ¼ cup of cheese. Put it back in the oven for a couple of minutes for the cheese to melt. Serve warm.