Make the miso butter by mashing the butter and miso together until you have a smooth, uniform paste.
Bring a large pot of well-salted water to a boil, and cook the spaghetti for 2 minutes less than the package directions state.
Add the olive oil and mushrooms to a large frying pan over medium heat and saute until the mushrooms are cooked through and there is no liquid in the pan.
Add the garlic and saute until the mushrooms have browned around the edges.
When the pasta is done, use tongs to transfer it directly to the pan with the mushrooms. Add a ladleful of boiling liquid along with the snap peas. Continue cooking the pasta like risotto, stirring constantly and adding more liquid as it evaporates for another minute.
Finish the miso pasta by adding the miso butter and vigorously mixing to create an emulsion with any remaining liquid in the pan. If the pasta starts getting dry, just add more pasta liquid.
Serve garnished with chopped Italian parsley.
