Well this is what I did (I think) and it was really yummy:
Filling: Melt 25g butter in pan. Add 50g plain flour and mix. Add 250ml chicken stock, whisk and cook through. Add 50ml milk/cream. In a separate pan I sweated 1 chopped onion and 1 leek. Add 250g chopped chicken and cook through. Add 250g asparagus (I think - forgot to measure it). When all cooked through I added to the sauce.
Then I lined a 25cm tin with shortcrust pastry and blind baked it. Put the filling in and a lid with holes for steam. Baked for 30 mins. I reckon it would feed 4-5.