Heat a large skillet over medium-high. Add a tablespoon of butter or olive oil and sauté the mushrooms (in two batches), season with salt and pepper, making sure not to crowd the pan. Sauté until lightly browned. Remove and set aside. Repeat with the second batch of mushrooms, adding more olive oil and/or butter as needed. Remove and set aside.
Heat a tablespoon of olive oil in a skillet over medium heat. Add the onion, season with salt, and sauté until lightly browned, about 5 minutes.
Add the reserved mushrooms to the skillet. Add the herbs and garlic. Add the tomato paste and stir 1 minute. Add the Sherry (or Madeira) and deglaze the pan, scraping any bits that stick to the bottom. Cook a couple of minutes until the Sherry is reduced.
Sprinkle in the flour and add the stock. Season with salt and pepper. Simmer over low heat, stirring from time to time, until the sauce is thick and reduced, about 10 minutes.
Garnish with chopped parsley.
To Finish the Dish: Bring a pot of salted water to a boil. Drop in the pasta and cook until al dente, aboout 2-3 minutes. Add the cooked pasta to the skillet with the mushroom ragú. Toss to coat the pasta, adding a little pasta water if needed to loosen the sauce. Divide amongst bowls. Serve topped with freshly grated Parmesan.