Bring a medium saucepan of water to a boil.
In the cup of a high-speed blender, add the rehydrated shiitakes, onion, garlic, soy sauce, beetroot and shiitake soaking water. Blitz in the blender but do not puree. It should resemble a chunky tapenade. Set aside.
In a large bowl, measure out the remaining ingredients. Mix in the wet, blended ingredients with a spatula until it all forms one big ball of “dough”. Tip out onto the work surface and knead for a good five minutes. The “dough” should be pretty wet - much wetter than ordinary meatball mix.
Form the “dough” into meatballs of your desired size (I went for walnut sized ones). Wrap each ball in tinfoil and twist at the top to keep it tight. Plop the meatballs into the boiling water and boil for 25 minutes.
Once boiled, remove the meatballs from the water and carefully remove the tinfoil. The meatballs should be soft but nicely bouncy to the touch.
Heat a large frying pan over a medium heat and add some olive oil. Fry the meatballs, stirring often until nicely browned. Tip the meatballs onto a plate lined with kitchen towel.
Throw the chunks of sourdough into the meatball pan (which should still have some oil in iand season with salt and pepper. Place over medium heat and toss regularly until nicely browned.
Serve a few good ladles of soup in a bowl. Top with a few meatballs, some croutons, a small bunch of torn fresh oregano and a drizzle of olive oil.