4 Portobello mushrooms, slicedÌý
8 garlic cloves, peeled but left whole
1 onion, thickly sliced
1 tbsp finely chopped fresh parsley
1 tbsp finely chopped fresh tarragon
3 anchovy filletsÌý
500ml/18 fl oz chicken stockÌý
20 prunes, stones removed
2 bay leavesÌý
salt and freshly ground black pepper
1 tsp fennel seeds
4 tbsp vegetable oil
splash white wine vinegarÌý
1 whole duck
4 good-quality bread buns
150ml/5fl oz white wineÌý