Chop each soaked but uncooked prune into about 8 pieces. Mix with the breadcrumbs and the onion in its butter. Season, using plenty of pepper.
Trim the pork fillets and cut across in halves. Split each half longways, and spread the centres with the prune filling, closing like sandwiches. Rub with seasoned flour and place in a flat ovenproof dish. Just melt remaining butter and pour over