Fill a large pot or dutch oven with salted water and bring to a boil.

Boil the noodles until they are al dente according to the package instructions. 

Drain the noodles, reserving a cup of pasta water.  Toss the noodles lightly in olive oil and set aside.

In the same pot, melt your butter and add the onions and mushrooms. Saute over medium-high heat until soft and golden brown.

Add the garlic and saute another 1-2 minutes, until fragrant. Stir regularly to prevent it from burning.

Add the flour and stir well to thoroughly coat the mushrooms in flour.

Pour the cream into the pan and give it a quick stir. Heat the mixture until it begins to simmer. Do not allow it to boil.

Add the cheese and continue to simmer until the sauce starts to thicken. Add pasta water as needed to achieve your desired consistency.

Add your spinach, then simmer until the spinach wilts, around one minute.

Toss the cooked pasta in the sauce and continue simmering until the pasta is warm and everything comes together. Do not overcook as your noodles will get mushy.

Add lemon juice and give the pasta a quick stir.

Serve immediately, topped with extra parmesan cheese.