Place the black fungus and shiitake into a suitable container, and pour over enough boiling water to cover. Set aside for 20 minutes.
While the mushrooms are rehydrating, prep the chanterelles and oysters, and wipe the button mushrooms. If they are larger than bite-sized, cut them in half.
Mince the ginger. You don't need to peel fresh ginger, BTW, especially not when the skin is very thin anyway.
Slice the spring onions, and set them aside.
Toast the sesame seeds for a couple of minutes in a dry skillet over a medium heat. Keep them moving so they don't burn. Set aside.
Place all of the sauce ingredients into a clean jar, screw on the lid, and give it a good shake. Make sure there are no lumps of cornflour. Set aside. (Alternatively, you can whisk them together.)
Once the black fungus and shiitake have plumped up, drain, and remove as much excess moisture as possible by wrapping in a clean tea towel or some kitchen roll, and gently blotting. Unwrap, and roughly slice the fungus. You can keep the shiitake whole if they are small, if not, cut in half. (Keep the broth as it's full of goodness, and makes a great base for stock.)
Place the wok over a very high heat, and once it begins to smoke, add the oil.
Immediately add the minced ginger, and stir-fry for 20 seconds or so.
Add the button mushrooms, and fry for a couple of minutes, then add all of the other mushrooms, and continue to stir-fry for 5-7 minutes.
Give the sauce a shake, and add to the wok. Stir-fry for another 3-4 minutes, until the sauce has thickened and become glossy (this is why it's best to use tapioca, not cornflour).
Turn off the heat, and stir in the sesame oil and all but a few of the spring onions.
Serve sprinkled with the toasted sesame seeds and the rest of the spring onions.
Leftovers can be kept in an airtight container in the fridge for a day. This recipe doesn't freeze well.
The mushroom mix you use is up to you, and if you're lucky enough to be able to get fresh jelly mushrooms (cloud ear, etc.) and shiitake, then by all means, use those instead. You'll need to use about 100g (3½ oz) of each. Porcini, brown boletus, and king oysters all work well together.