For the stuffing, heat three tablespoons of the oil in a large, non-stick frying pan. Tear the bread into small cubes and fry over a medium heat for 4-5 minutes, or until pale golden-brown. Add the chopped rosemary and cook for one minute. Remove the mixture from the pan using a slotted spoon and place into a bowl.
Return the pan to the heat and toast the hazelnuts for 1-2 minutes, then tip into the bowl with the bread.
Melt half of the butter with the remaining oil. Add the finely chopped onion and fry gently for three minutes, stirring occasionally.
Add the apples to the pan, sprinkle with the sugar and fry for 3-4 minutes over a medium heat until softened and lightly browned. Add the garlic and cook for a further minute, stirring constantly.
Remove from the heat and add to the bread, along with the orange zest and parsley. Season with salt and freshly ground black pepper and mix until well combined. Set aside to cool.
Place the remaining onion in three piles, evenly spaced apart on a sturdy roasting tin or deep baking tray. Spoon the stuffing into the cavity of each duck.
Place the ducks on top of the sliced onion, breast-side facing upwards. Cover with the remaining butter and season with freshly ground black pepper. Place the bacon rashers in a cross on top of each duck. Roast for 35 minutes, or until golden-brown. Remove the ducks from the roasting tin and cover with foil and tea towels.
Place the roasting tin on the hob and stir in the flour. Cook for a minute and then slowly stir in the sherry.
Return to a simmer and cook for a further 2-3 minutes, stirring regularly. Pour any juices that have collected under the ducks into the pan. Pass the sauce through a fine sieve into a small saucepan and season to taste with salt and freshly ground black pepper.
Remove the stuffing gfrom the ducks with a long spoon and divide between warmed plates. Slice the breast on each side at an angle of each duck using a sharp carving knife, working between the wing and the leg.
Cut off each leg and place at an angle to the breast. Garnish with fresh rosemary. Repeat with the other duck. Spoon over a little of the sauce and serve.