01 -
Bring a medium pot of water to a boil over high heat. Cook the pasta according to package directions. Reserve 1/3 cup cooking water, then drain and return the pasta to the pot.
02 -
Heat the olive oil in a large skillet over medium heat. Add the mushrooms and cook, stirring every 2 minutes, until browned and softened, about 10 minutes.
03 -
In a small bowl, whisk miso into the reserved cooking water until combined. Add the miso mixture, mushrooms, butter, garlic, pepper, and salt to the pasta. Stir gently until coated in a creamy sauce. Top with scallions if desired.