Cook the noodles in a large pot per the instructions on the package, drain the pasta water, and set the cooked pasta aside.
In a large skillet, add the butter and onions. Then add the mushrooms. Cook over medium heat until onions are translucent and the mushrooms are lightly browned.
In a large skillet, add the sausage and cook over medium heat with a wooden spoon until it starts to cook through and lightly brown. Turn heat up to medium/high and brown and continue to cook until done, this will only take about 5 to 7 minutes. Remove from heat and set aside.
In large serving dish, add the noodles, sausage, onions and mushrooms and toss completely.
To serve: Place a bed of spinach in each bowl, add hot pasta on top, 1 to 2 tablespoons of butter, and stir. The hot pasta will cook the spinach as you stir and combine the butter.
Grate fresh parmesan on top and salt and pepper to taste.
Store leftovers in the fridge for up to 5 days.