Start by cooking the pasta of your choice according to the package directions. Drain and set aside.
Using the same pot used to cook the pasta, add the olive oil and heat over medium heat. Once the oil is hot, add the shallots, garlic, and crushed red pepper. Cook while occasionally stirring for 3-4 minutes until fragrant. Next add the sliced mushrooms and cook until softened (3-4 more minutes).
Turn the heat up to medium high, then add the Italian sausage and break it up into small bite-sized pieces while it browns. It should take 5-7 minutes for the sausage to fully cook.
Add the baby spinach and lemon juice, stir to combine with the rest of the ingredients until the spinach is wilted (2-3 minutes).
Add the heavy cream, grated parmesan, and salt and pepper to taste. Stir everything together well, and let the liquid come to a soft boil. Simmer for a couple minutes to allow the cheese to melt and the sauce to slightly thicken.
Add the cooked pasta into the sausage mixture, stir to combine, and serve immediately with extra grated parmesan.