Preheat smoker or oven to 350F (I use 300F for a Gateway Drum Smoker).
In a 9 inch (or smaller) pie dish or heat-proof casserole dish, spray dish with non-stick spray. Lay oysters to cover the bottom of the dish. Set aside.
In a frying pan on medium heat, melt 1 tablespoon of butter and add garlic, onion, celery, green pepper, or/and carrot and cook until softened.
Add remaining butter to melt and spinach to warm.
Add white wine to deglaze frying pan. Set aside to cool.
Meanwhile, in a medium bowl, add breadcrumbs, 2 tablespoons parmesan cheese, Japanese mayonnaise, salt and pepper.
Add spinach mixture from frying pan into bowl of breadcrumbs.
Toss mixture to combine. Add additional salt and pepper as required.
On top of the oysters in the casserole dish, evenly spread the spinach mixture.
Sprinkle additional 2 tablespoons of grated parmesan cheese.
Place oyster casserole dish into smoker or oven. Smoke or bake for 40 minutes.