Heat a large skillet or cast-iron pan over medium heat on the stove and render down the pork fatback (drain off 75% of the fat and discard).
Add the onion and garlic to the pan with the pork fat and sauté for five to ten minutes, or until onions start to become translucent.
Add the white wine, potatoes, thyme and bay leaf. Place the cod on top and cover.
Steam until the cod starts to flake apart slightly, about ten to fifteen minutes.
Remove the lid and incorporate the butter then the béchamel. Check for seasoning and gently stir in the fresh herbs.
For the Bechamel:
In a medium saucepan add the butter and flour and cook together for two minutes. Slowly pour in the milk whisking constantly.
Cook until the sauce begins to thicken. Add the lemon zest, Dijon mustard, savoury and season with salt and pepper to taste. Add the Parmesan cheese and mix in.
To assemble:
Put a thin layer of fish mixture into individual gratin dishes. Top with sprinkle buttered breadcrumbs and and then some shredded cheese some more fish mixture.
Meanwhile, mix the cracker crumbs with the savory and olive oil and sprinkle over the cheese.
Either freeze or bake for 20 minutes at 350 degrees F until bubbling and the top has evenly browned. If frozen, bake for 30 min or until the fish is warm.