Pork sausage and apple pie

Ingredients

• 1 tablespoon olive oil
• 1 medium_piece (150g) brown onion, chopped finely
• 1 (130g) baby fennel bulb, sliced thinly
• 6 (720g) thick pork and herb sausages
• 2 teaspoons plain (all-purpose) flour
• 1 cup (250ml) chicken stock
• 2 teaspoons mild english mustard
• 1 medium_piece (150g) apple, grated coarsely
• 2 tablespoons fresh flat-leaf parsley, coarsely chopped
• 1 egg, beaten lightly
• Pastry
• 1 1/2 cup (225g) plain (all-purpose) flour
• 80 grams cold butter, chopped coarsely
• 1 egg yolk
• 2 tablespoons iced water, approximately