Add a medium pot (4-5l capacity) with the water to boil with the chicken breast cut in half (skin and bones too).
Add the washed mushrooms (portobella mushrooms can be sliced if you like them that way), the chopped carrot and red bell pepper, the soy sauce and the chicken stock cube.
Leave the pot to simmer for 25 minutes, then take out the chicken breast with its skin and bones.
Take the meat out of the bones and re-add it to the pot, along with the red thay paste and the baby spinach.
Add the rice noodles right after and leave to simmer until the noodles are soft.
Turn off the heat and set aside for 10 minutes.
Mix the miso paste in a separate bowl and add gradually some of the soup and mix thoroughly until a homogeneous composition is obtained.
Pour the miso to the pot and mix together.
Serve with fresh spring onion slices on top.