1 For the vol au vent cases: mix together the potato starch, cornflour, brown rice flour and sweet rice flour in a mixing bowl and whisk to combine.
2 Add the xanthan gum, guar gum and salt and stir well.
3 Add the frozen butter and using an electric whisk mix on the lowest speed, until the edges of the butter pieces have started to soften.
4 Pour in the water and mix until absorbed.
5 Turn out the mixture onto a large sheet of non-stick baking paper and knead it together with your hands, just enough to bring it together.
6 Place another sheet of baking paper on top and gently roll the dough out to a rough rectangle, taking care not to roll over the edges.
7 Fold the bottom third of the dough towards the middle, then fold the top third on top of it.
8 Rotate the dough one-quarter turn to your right (clockwise).
9 Roll out the dough gently as before, until smoother and more cohesive. Fold the bottom third up and overlap the top third over it.
10 Rotate the dough one-quarter turn to your right (clockwise).
11 Repeat the turning, rolling and folding process twice more.
12 Wrap the folded dough in cling film and chill for 2 hours.
13 Remove from the refrigerator and stand at room temperature for about 20 minutes.
14 Repeat the turning, rolling and folding process twice more. Wrap the dough in cling film and chill for a further 2 hours, then stand at room temperature for about 20 minutes before using.
15 Heat the oven to 200°C (180° fan) 400°F gas 6. Line a baking tray with non-stick baking paper.
16 Roll out the pastry about 5mm|1/4" thick. Use a cookie cutter to cut roughly 5cm|2" diameter discs. Place on the baking tray. Use a slightly smaller cutter to mark smaller circles inside each of the discs, but don't cut all the way through to the base of the pastry.
17 Brush the pastry with beaten egg. Bake for 10-15 minutes, until puffed and golden brown. Carefully remove the tops and centres, so you are left with little pastry cases. Return to the oven for 2-3 minutes to dry inside.
18 For the filling; heat the butter in a frying pan. Add the onions and cook gently for 2 minutes.
19 Add the garlic and cook for 1 minute, then add the crayfish and cook for 2 minutes, turning from time to time.
20 Stir in the cream, bring to a simmer and cook for 1 minute.
21 Stir in the dill and lemon juice then remove from the heat. Spoon the mixture into the pastry cases Serve warm garnished with chives.