1.Make shortcrust pastry. Process flour and butter until crumbly. Add egg yolk and most of the water, process until ingredients just come together. Knead pastry on floured surface until smooth, enclose with plastic wrap, refrigerate 30 minutes.
2.Preheat oven to 200°C (180°C fan forced). Oil 1.5-litre (6-cup) ovenproof dish.
3.Heat oil in large frying pan, stir onion, fennel and mince squeezed from sausages until browned. Add flour, cook, stirring, until mixture bubbles and thickens. Gradually stir in stock and mustard, stir over heat until mixture boils and thickens, simmer, 5 minutes. Stir in apple and parsley season. Spoon mixture into dish.
4.Roll pastry between sheets of baking paper until large enough to cover dish. Top dish with pastry, brush with egg. Bake about 30 minutes or until pastry is browned.