Trim the shiitake mushroom stems and separate the caps and stems. Slice the caps thinly and cut the stems in half lengthwise. Slice the onion thinly.

Cut the bacon into 1cm wide strips.

Boil a pot of water, add salt (not listed in ingredients), and cook the spaghetti according to the package instructions. Reserve 4 tbsp of the pasta cooking water before draining.

Heat olive oil in a frying pan over medium heat. Add the bacon and cook until coated in oil. Add the onion and shiitake mushrooms and sauté until softened.

Add the cooked spaghetti to the pan and mix well. Add the reserved pasta water and soy sauce, stirring to combine. Add butter, salt, and pepper, and stir until the butter melts and blends in.