Build the broth: Add the miso paste to the boiling water in a saucepan and whisk until dissolved. Add the grated ginger, sliced shiitake mushrooms and soy sauce. Let it simmer gently for 2 minutes to infuse.
Poach the egg: Crack the egg into the broth and poach for 2 to 3 minutes until the white is set but the yolk is still soft.
Assemble the bowl: Place the sliced portobello mushroom in a serving bowl. Add the egg noodles and spinach. The hot broth will heat the noodles through and wilt the spinach.
Serve: Spoon the broth into the bowl over everything. Lift the poached egg on top. Scatter with chopped spring onions.
