Melt the butter in a frying pan and add the onion. Cook gently for five minutes. Add the spinach and mix well. Season with some salt and pepper.
Divide the spinach and onion mixture amongst the oysters.
Top with the crispy bacon.
Mix the parmesan, breadcrumbs and the parsley together and then sprinkle this over the top.
Heat your grill to high and place the oysters underneath. Cook until bubbling and golden brown.