Peel and finely chop the onion. Melt the butter in a pan and fry the onion gently it is soft and translucent (about 10 minutes). Stir in the sage and the breadcrumbs then use this mixture to stuff the prunes (you're just filling the space where the stone came from).
Stretch the bacon a little using the back of a knife, then cut them in two and use each piece to wrap round a prune. Secure the bacon with a wooden cocktail stick then grill (broil) for about 4-5 minutes on each side until the bacon is crisp.