Preheat the oven to 180°C/Gas 4. Make the usual shortcrust pastry ( or use ready made) and bake blind then dry out in the oven.
Meanwhile, make the filling: heat a frying pan over a medium heat, add the oil when the pan is hot and then slosh in the chopped onion, apples, celery and sage leaves, season and sauté until beginning to turn translucent and soften.
Crumble in the sausages and fry a little longer, just to begin browning them.
Remove the tart from the oven and spread the mustard over the base – if you like mustard – then scrape the contents of the pan into the tart shell and pat down gently.
Return to the oven for 35–40 minutes or until browned and cooked through.
Remove from the oven and cool for 15 minutes before turning out and serving.