Preheat oven to 180ºC / 350ºF / Gas Mark 4. Place potatoes into a pan of boiling water and cook until tender.
Heat oil in a large pan, add sausages and leeks. Cook until sausages are lightly browned.
Add the parsnips and flour, mix well. Add the cider and stir until all the flour has mixed into the liquid and started to thicken slightly. Add sage leaves. Spoon into a shallow ovenproof dish.
When tender drain potatoes, mash with a splash of milk and knob of butter. Add half of the cheese and stir through.
Place in large spoonfuls onto the sausage mixture, but leaving some of the sausages and leeks to poke through (they will brown nicely if exposed). Sprinkle with remaining cheese and bake in a preheated oven for about 30 minutes until the top is golden brown.
Serve with additional seasonal green vegetables.