Mix flour and mustard together and rub well into the lean surfaces of the pork loin.
Place in a roast pan and add the beer.
Roast 15 minutes in a hot 450F oven then lower the heat to 350F
Remove the centres of the onions and stuff with the prunes.
Arrange around the roast and bake for 45 minutes, basting onions often with the ale.
Continue roasting until pork is cooked.
Lift roast and onions to a serving platter.