Bring a large pot of salted water to a rolling boil, then add the pasta and cook until just al dente, about 8 minutes. Drain and set aside, reserving a cup of pasta water.
While the pasta cooks, heat 2 tablespoons of olive oil in a large skillet over medium heat. Add the sliced mushrooms in a single layer, letting them sit untouched for 2 minutes until they start to brown.
Stir the mushrooms and continue cooking, stirring occasionally, for about 8 minutes, until they are deeply caramelized with a smoky aroma and dark golden edges.
Push the mushrooms to one side of the skillet. Add the sliced garlic to the cleared space and cook for 30 seconds until fragrant and golden, stirring constantly to avoid burning.
Mix the garlic with the mushrooms, then pour in a splash of reserved pasta water if the pan looks dry. Season with salt and pepper, cooking for another 2 minutes until the mixture thickens slightly.
Add the spinach in batches, stirring until it wilts quickly and turns a bright, vibrant green—about 2 minutes. Adjust seasoning as needed.
Turn the heat to low and add the drained pasta directly into the skillet. Toss everything together, adding more pasta water if needed to loosen the sauce and coat the pasta evenly.
Sprinkle the freshly grated Parmesan cheese over the pasta, then squeeze in the lemon juice. Toss again to combine and season to taste with additional salt and pepper if desired.
Serve immediately, garnished with extra cheese or a drizzle of olive oil if you like. The dish should look glossy, with caramelized mushrooms nestled among tender greens and al dente pasta.
