I did this with canned salmon, and it turned out fairly well. I wasn't able to reserve any sauce, since the fish was mixed in with it, so there wasn't really a sense of a sauce on the finished dish. Also, I wilted the spinach slightly before layering it over the fish mixture, just for convenience in handling.
This was a fantastic dinner! I substituted the spinach with asparagus and served it over garlic red mashed potatoes. My husband was pleased, too.