Step 1
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.
Step 2
Meanwhile, cook the sweet potato in a medium saucepan of boiling water for 5 minutes or until tender. Drain well.
Step 3
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.
Step 4
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.
Step 5
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.