Preheat the grill to its highest setting.
Open the oyster shells with a shucker. Discard the top and loosen each oyster from the base of its shell. Set aside and keep cool.
Melt the butter in a small frying pan. Fry the onion and the parsley for 2-3 minutes, until softened.
Add the spinach and cook until wilted.
Add the breadcrumbs and cook for 1-2 minutes.
Place a teaspoon of spinach and breadcrumb mixture onto each half oyster shell to cover the oyster and place a small lump of butter on top of each.
Place the oysters onto a baking tray and transfer to the grill to cook until golden-brown.