In a large bowl rub the butter into the flour and salt with your fingers until the mixture resembles breadcrumbs.
Stir in the yoghurt with a flat bladed knife until the mixture starts to form into clumps. Start with 3 tbsp, then use more if needed.
Cover and leave to rest in a cool place for ½ an hour or so.
Roll the pastry out on a floured surface as thinly as possible and cut out circles big enough to fit the tart cases. Reroll the trimmings until all of the pastry is used.
Press the pastry into the sides and trim the tops with a knife.
Blanch the asparagus in a little simmering water for a couple of minutes. Drain the spears and cut theminto three pieces.
Spread a ½ tsp of pesto on the bottom of each tart case.
Place the asparagus pieces over the top.
Whisk the egg (or eggs) in a jug with the milk and a little salt and pepper and divide the mixture between the tarts. Be careful not to overfill them as the mixture will rise during cooking.
Bake at 200℃ (400℉, Gas 6) for about 20 minutes or until the pastry looks golden and the filling has set.
Leave to cool in the cases then remove the tarts from the tins.
You can use a food processor to make the pastry.There will be some pastry left over, enough for eight small jam tarts.The asparagus water makes lovely stock.The tarts can be kept in the fridge for up to three days.Please note: calories and other nutritional information are per serving. They're approximate and will depend on exact ingredients used.