Put the dried mushrooms in a large, heavy-based saucepan and cover with the water. Add the soy sauce, stock cube and miso paste and stir until the stock cube has dissolved. Set aside for 30 minutes to rehydrate and infuse.
Meanwhile, half-fill a saucepan with water and bring to the boil. Add the noodles and cook for 3-4 minutes, or until just tender, stirring occasionally to break up the strands. Drain well, then rinse under running water until cold.
Half-fill the same pan with water and bring to the boil. Add the eggs to the boiling water and cook for 5 minutes.
Heat 2 teaspoons of the oil in a large, non-stick frying pan over a medium heat. Add the tofu pieces and fry carefully for 3-4 minutes on each side, or until pale golden-brown on both sides. Turn off the heat but keep the pan on the hob so that the tofu stays warm.
Stir the noodles into the mushroom broth and bring to the boil. Reduce the heat until the mixture is simmering, add the chestnut mushrooms and pak choi and continue to simmer for a further 2-3 minutes. Stir in the beansprouts and spring onions, remove from the heat.
Drain the eggs then rinse under cold water. Crack and peel off the shells, cut in half.
Divide the broth, noodles and vegetables between 4 serving bowls. Top with the tofu and eggs. Sprinkle over the cashew nuts and chilli flakes. Season with extra soy sauce.