3. Step
Reduce heat to medium; add the half-and-half mixture to the mushroom mixture. Cook, stirring constantly, until the miso breaks down, about 1 minute. Add spinach and Parmesan; cook, stirring constantly, until the sauce slightly thickens and the spinach wilts, about 2 minutes. Add the pasta; toss gently to coat, adding more reserved cooking water if needed to reach desired consistency. Remove from heat and stir in lemon juice.