Preheat oven to 400˚. Cook pasta on stovetop for 5 minutes then drain. Reserve half cup of pasta cooking water before draining. Add spinach just before draining to thaw. If using fresh you can flash cook the spinach using the same method. Combine remaining ingredients, including reserved water, save for 1/4 cup feta and mozzarella cheeses. Pour mixture into 3-quart baking dish (9x13 inch) and top with remaining cheeses. Bake for 25 minutes covered. Uncover and bake an additional 10 minutes to melt cheese topping.