Bring a medium pot of water to a boil over high heat. Cook pasta according to package directions. Reserve ⅓ cup cooking water; drain. Return the pasta to the pot.
Meanwhile, heat oil in a large skillet over medium heat. Add mushrooms; cook, stirring about every 2 minutes, until browned and softened, about 10 minutes.
Whisk miso into the reserved ⅓ cup cooking water in a small bowl until combined. Add the mushrooms, the miso-water mixture, butter, garlic, pepper and salt to the pasta in the pot; gently stir until the pasta is coated in a creamy sauce. Divide the pasta mixture among 4 bowls. Top with scallions, if desired. Additional reporting by Carrie Myers, M.S. and Jan Valdez
