Cut the mushroom stems crosswise into 1-inch pieces, you should have about 8 pieces total (reserve the caps for another recipe.), then use a paring knife to score both flat sides in a cross-hatch pattern.
Whisk the stock, white wine, miso, soy sauce, brown sugar and Worcestershire sauce in a small sauce pan over low heat, add the mushrooms, and simmer for 10 minutes. Remove the pan from the heat and allow the mushrooms to marinate in the mixture for a further 10 minutes, then discard the marinade, remove the mushrooms with a slotted spoon and pat them dry with paper towels.
Bring a large pot of lightly salted water to a boil, and as soon as the water boils, fold in the pasta and cook until al dente, about 7 minutes for linguine. Remove a ladle of pasta cooking water from the pot and set aside.
Heat the butter and extra-virgin olive oil, a sprig of thyme and garlic clove in a large non-stick skillet over medium heat, add the mushrooms and cook them for 3 minutes on each side, or until nicely browned on both sides. Discard the garlic and thyme sprig.
Add the spinach into the pan, followed by the lemon zest, cover with a lid, and reduce the heat to a minimum, then cook for 2 minutes stirring occasionally.
Remove the pot from the heat and drain the pasta. Transfer the pasta into the skillet withe the mushrooms and spinach, add some of the reserved pasta water and optional creme fraiche, and stir all the ingredientsTop the pasta with freshly-crackedblack pepper and serve.