In a mixing bowl, add miso paste + warm water. Mix well until the miso dissolves. Set aside.
Set a large pot over medium heat. Add oil.
Once it's hot, add garlic + onion and stir-saute for 1-2 minutes, or until the onion's translucent.
Next, add the mushrooms + a pinch of salt + pepper, and cook for about 10 minutes, or until much of the liquid is released and evaporated, and the mushrooms turn dark.
In the meantime (while mushroom is being cooked), prepare the spaghetti in a large saucepan using package instructions, until al dente.
Once the mushrooms are done, add flour and mix well for about 1 minute, until it absorbs all the water in the pot.
Now, add soy milk + salt + pepper + nutritional yeast. Stir well and make sure there aren't any lumps. Bring it to a boil.
Once it comes to a boil, lower the heat and add the cooked spaghetti. Gently fold and mix - allowing the spaghetti to be nicely coated with the sauce.
Turn off the heat and add the prepared miso water + freshly sliced green onions.
Give everything a gentle mix. Creamy miso mushroom pasta is ready! :)
Notes
I added the miso at the end to preserve its umami taste and nutrition.
IMO, green onions and black pepper are the secret stars of this recipe. So don't hold back while adding them :D