Combine flours, psyllium and salt in a large bowl. Whisk in the oil and enough boiling water to form a damp but crumbly dough. If in doubt, err on the side of caution – you can always add more water later.
Turn the mixture out onto a clean, dry bench top. Allow it to cool for a minute or two before beginning to knead with your hands. It will be hot, so be really careful. Continue kneading until a smooth and pliable dough forms. If it doesn’t, add half a tablespoon of hot water at a time until you reach a good consistency.
Tear off a golf-ball-sized piece of dough and cover the remainder. Roll dough out on a well-floured surface (I use tapioca flour) until it is as thin as you want it, keeping in mind that it swells a little during cooking.
Use a medium-sized circular cookie cutter (or an upside-down glass) to cut out rounds of dough. Mine is 9.5 cm. I like to fill and shape the dumplings as I work, but you can roll them all out before filling if you prefer. Just make sure to flour in between the rolled-out wrappers well and keep them covered if you choose the latter option.
Re-roll any excess dough as you work. You might need to moisten trimmings with wet fingers to return it to its pliable glory. Repeat with the remaining dough until it is all used.
Fill your dumpling wrappers with a teaspoon or two of your filling of choice. Use water to thoroughly seal the dumpling edges together in any sort of pleating style you like. I like to really squish the edges together to ensure the dumplings won't split upon cooking. Repeat until you have filled all the dumplings.
To cook, boil a medium pot of lightly salted water. Once it is boiling, reduce it to a simmer and add 5-6 dumplings to the water. Stir very gently to ensure they don't stick to each other, and reduce the heat if it returns to a rolling boil at any point.
Cook each batch of dumplings for 2-3 minutes or until the wrapper starts to look a bit translucent. Remove from the water and place into a strainer to cool. Very lightly oil each batch of dumplings with toasted sesame oil to ensure they don't fuse together. Repeat until you have cooked all the dumplings you need.