Take the pastry out of the fridge 30 minutes before making the recipe.Preheat the oven to 425 F/ 220 C/ gas mark 7. You will need 5-6 ramekins/mini pie dishes.
To make the sauce in a saucepan melt the butter, add the flour and quickly whisk over a low/medium heat until the mixture becomes smooth. Add half of the milk, increase the heat and cook until the mixture thickens whisking all the time. Pour in the rest of the milk and continue cooking and whisking until the sauce starts bubbling up. Remove from the heat, add the mustard, seasoning and fromage frais/fromage blanc (or alternative) and stir until smooth.
In a large bowl combine the sauce with the chicken and asparagus. Spoon the mixture into your oven dishes stopping about 0.5 cm from the top.
Unroll the pastry and cut out squares or circles. The shapes can be, but don't have to be a little bigger than your dish. Place the pastry shapes over the pies pressing gently to seal along the rim. Using a pastry brush thinly coat the top of the pastry with the egg mixture. Place the mini pies on a large baking sheet leaving plenty of room in between and bake in the centre of the oven for 25 minutes. If the pies start to brown a little too much (after 15 minutes or so) cover them loosely with a large sheet of tin foil.
Remove from the oven, set aside for 10 minutes, then serve.
Do not fill your dishes with the chicken mixture right up to the top as the sauce may spill out during cooking.
Remove the pastry from the fridge about 30 minutes before making the pies.
You can trim the pastry to fill your dish or cut it into larger squares as I have done.
Use a mixture of dark and white meat or just dark meat. White meat alone might be a little dry.
I recommend using asparagus tips that aren't too thick or thin. Medium thickness in best.
Use chicken stock instead of the milk if you prefer (if you make this swap you will probably not need to add any salt into the sauce).
Brush the pastry with only a very thin coating of the egg mixture.