Rinse, clean and pry open the oysters; discard top half shell; gently run a table knife under the oyster, releasing it from the bottom shell, but leave it in place. Set these aside.
Melt butter in a medium saucepan over medium heat; add onion and sauté ~4 minutes.
Add the spinach and sauté until wilted. ~2 minutes.
Add wine, lemon juice, cream, cheese and pepper; bring to a bubbly boil; reduce heat to low; continue cooking until the liquid has reduced by half, ~5 minutes.
Refrigerate this mixture, until cooled and solidified.
Preheat oven to 375°F; place oysters/shells on a broiler pan, or other oven safe dish; spoon the mixture on top of each shell, covering the oyster and shell; top each with panko bread crumbs.
Bake for ~20 minutes, until the topping is golden brown.
