Heat the extra virgin olive oil in a sauté pan over high heat until it's smoking. Add the chopped onion and garlic, and cook for about 5 minutes until the onions are lightly golden brown.
Add the sliced mushrooms to the pan and continue cooking until they release most of their water, about 9-10 minutes.
Pour in the dry white wine (or broth), and add the freshly chopped thyme, salt, and freshly-ground black pepper to taste. Reduce the heat to medium and cook for an additional 5 minutes until the mixture reaches a ragù-like consistency.
In a separate large pot, bring 6 quarts of water to a boil, adding 2 tablespoons of salt. Cook the pasta until it's al dente, then drain, reserving some of the cooking water.
Add the drained pasta to the mushroom ragù in the pan, and toss to combine.
Add the fresh lemon juice and chopped parsley to the pasta mixture. Toss over high heat for 1 minute to evenly coat the pasta. If needed, add a ladleful of pasta water to achieve the desired sauce consistency.
Taste and adjust the seasoning with salt and freshly-ground black pepper as necessary.
Divide the pasta into four warmed pasta bowls. Top each serving with freshly grated Parmigiano Reggiano.
Serve the Pappardelle with Mushroom Ragu immediately and enjoy the delightful blend of flavors and textures.