Bring a large pot of salted water to a boil, then add the pasta and cook until just al dente, about 8 minutes. Reserve a cup of pasta water, then drain the pasta and set aside.
While the pasta cooks, heat olive oil in a large skillet over medium-high heat. Once shimmering, add the sliced mushrooms and sauté, stirring occasionally, until they develop deep golden brown edges and become fragrant, about 8-10 minutes.
Add the finely chopped garlic to the mushrooms and cook for about 30 seconds, until fragrant and slightly toasted—your kitchen will fill with a warm aroma.
Toss the fresh spinach into the skillet in batches, stirring until each addition wilts and turns vibrant green, about 2 minutes. The greens should be tender but still bright in color.
Lower the heat to medium, then add the drained pasta to the skillet. Pour in a splash of the reserved pasta water and toss everything together until the pasta is glossy and well-coated with the mushrooms and greens.
Squeeze fresh lemon juice over the pasta, then sprinkle generously with grated Parmesan cheese. Toss again to evenly distribute the flavors and create a shiny, flavorful coating.
Transfer the pasta to serving bowls, garnish with additional Parmesan if desired, and enjoy immediately while hot, with the aroma of garlic, mushrooms, and fresh greens filling your kitchen.
