Completely defrost the spinach and squeeze out any excess liquid. Add the butter or olive oil to a skillet and saute the scallions about 2 minutes. Add the spinach and continue to cook about 5 minutes longer. Add the sesame oil, cheddar, and mozzarella, folding the spinach over the cheese until it melts. Season with salt and pepper. Remove the filling from the pan and cool before stuffing your blintz crepes.
If you refrigerate the mixture, it is best to bring the spinach to room temperature, and drain again before stuffing or your blintzes will be soggy.