Heat oven to Gas 4, 180°C, fan160°C. Brush a 36x12cm (14x5in) loose-bottomed flan tin with a little melted butter.
Roll out the pastry, line the flan tin and place in the fridge to chill for 10 minutes. Mix together the quark, egg and milk until blended and smooth. Cut the asparagus and tuna into lengths that fit the width of the tart case.
Pour in the quark mix and alternately arrange the tuna and asparagus evenly in the case. Place in the lower part of the oven and bake for 30 minutes; reduce the temperature to Gas 3, 170°C, fan150°C, and cook for a further 10 minutes.
Serve with green salad leaves.