Boil pot of water and salt generously. Cook spaghetti accordingly to package instructions, usually around 10 minutes.
Wash shiitake thoroughly, especially under the caps. Prep by removing the stems and slicing the head into thin pieces.
Add 1 tbps of butter to a heated pan and coat evenly. Add the minced garlic and cook until slightly browned. Then, add the sliced mushrooms. Don't crowd the pan. Once the bottom has slightly browned which should take about 2-3 minutes, flip mushrooms and pan-fry the other side until golden brown, too. Salt and pepper the mushrooms, give them a good stir and remove from pan.
Let the pan heat up again and add 2 tablespoon of butter. Once melted, add in the miso paste and pasta water. Stir vigorously until miso has dissolved. Add in pecorino romano and stir until melted.
Add the cooked sliced mushrooms back into the pan and give it another stir so the mushrooms are coated.
Add the cooked spaghetti into the saucepan. Toss pasta until thoroughly coated.
Remove from heat and serve with more cheese and cracked pepper, to taste.