Preheat oven to 200°C. Heat the oil in a non-stick frying pan over medium heat. Add the onion and cook, stirring occasionally, for 15 minutes or until soft and golden. Add the sugar and cook, stirring, for 2 minutes or until onion caramelises. Add the mustard and stir to combine. Remove from heat.
Meanwhile, cook the sweet potato in a medium saucepan of boiling water for 5 minutes or until tender. Drain well.
Line a round 22cm (base measurement) fluted tart tin, with removable base, with pastry and trim any excess. Place in the fridge for 15 minutes to rest.
Cover the pastry base with non-stick baking paper and fill with pastry weights, rice or dried beans. Bake in preheated oven for 15 minutes. Remove paper and weights, rice or beans. Bake for a further 10 minutes or until golden. Remove from oven. Reduce oven temperature to 180°C.
Cover pastry base with caramelised onion and top with sweet potato. Whisk together the cream, egg and basil in a jug. Pour into pastry base. Bake in oven for 40 minutes or until golden and set. Serve with, mixed green salad leaves or coleslaw.