Put the seitan into a bowl with the soy sauce, garlic and ginger. Stir to coat the seitan, then set aside for 30 minutes to marinate.
Meanwhile, cook the noodles as per the packet instructions and set aside.
Once the seitan has marinated, pop into a saucepan and add the stock. Bring to a boil, then reduce the heat and simmer for 10-15 minutes, or until fragrant. You can add a little more water here if needed.
Meanwhile, heat a grill pan (or a frying pan with a little oil) and cook the mushrooms for 5 minutes, or until soft. Push to one side of the pan and add the pak choi, inside face down. Cook for 3-4 minutes or until the pak choi starts to char. Flip and cook for a further 3-4 minutes. Remove from the heat and set aside.
Add the miso paste to the stock and stir until dissolved. Then add the noodles, mushrooms, pak choi, carrot and spring onions and bring back to a simmer. Divide between two bowls and top with a pinch of chilli flakes if desired.