My roasted duck 'sorta shepherd's pie'

INGREDIENTS

• 1.5 kg Gressingham duck, giblets removed
• 1 orange, halved
• a few sprigs fresh rosemary
• a few sprigs fresh sage
• sea salt
• freshly ground black pepper
• olive oil
• 1 red onion, peeled and thickly sliced
• 2 sticks celery, cut into 5cm lengths
• 3 carrots, peeled and cut into big chunks
• 1 tablespoon plain flour
• 2 sticks rhubarb, roughly chopped
• 500 ml organic beef stock
• 1 kg potatoes, peeled and chopped
• 500 g swede, peeled and chopped
• 2 tablespoons butter
• 1 splash warm milk
• 1 clove garlic, peeled
• zest of 1 orange
• 1 handful Parmesan cheese, freshly grated