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, reserving 1/4 cup of the pasta water, and set aside.
While the pasta cooks, heat olive oil in a large skillet over medium heat until shimmering and fragrant. Add the finely chopped garlic and cook for about 30 seconds, until it becomes fragrant and slightly golden.
Add the sliced mushrooms to the skillet and sauté, stirring occasionally, until they release their moisture and turn golden brown, about 5-7 minutes. You should hear a gentle sizzle and see the mushrooms develop a deep color.
Push the mushrooms to one side of the skillet, then add a splash more olive oil if needed and toss in the chopped spinach. Stir until the spinach wilts down and turns vibrant green, about 2-3 minutes. The greens should look tender but still bright.
Add the drained pasta to the skillet with the mushrooms and spinach. Pour in the reserved pasta water, stirring gently to combine everything into a cohesive, slightly saucy mixture. Let it cook for another 2 minutes until the flavors meld and the dish is heated through.
Squeeze the fresh lemon juice over the pasta and sprinkle with grated Parmesan cheese. Toss everything together until well coated, tasting and adjusting seasoning if necessary. The lemon adds brightness that balances the earthiness of the mushrooms.
Remove from heat and serve immediately, garnished with extra cheese or lemon zest if desired. Enjoy the tender pasta coated in a fragrant, earthy sauce with vibrant greens and golden mushrooms.
