Preheat oven to 200°C. Line a 15cm round (base measurement) by 5cm-deep fluted tart tin with removable base with the pastry and trim any excess. Place in the fridge for 15 minutes to rest.
Cover pastry with baking paper and fill with pastry weights. Bake for 10 minutes. Remove paper and weights and bake for a further 6-8 minutes or until golden. Reduce temperature to 160°C. Spread the base of the pastry evenly with the mustard.
Melt butter in a large frying pan over medium heat. Add the leek and cook, stirring occasionally, for 5 minutes or until leek softens. Remove from heat.
Reserve 3 asparagus spears; coarsely chop remaining spears. Combine salmon, leek and chopped asparagus in a bowl. Spoon into pastry case. Whisk eggs, cream, dill and chives together in a bowl. Pour over salmon mixture. Bake in oven for 45 minutes or until filling is just set
Place remaining asparagus spears in a shallow heatproof bowl. Pour over boiling water and set aside for 2 minutes to soak. Drain well. Place on the quiche. Serve warm or at room temperature.