Boil water for the pasta. Cook pasta according to package instructions.
Melt the butter in a large skillet over medium-high heat and swirl in the olive oil. Add the mushrooms and onion and cook for 5-6 minutes, or until the mushrooms release their water, and it's cooked off. Take the mushroom mixture out of the pan and set them aside.
If the skillet looks dry, add a little olive oil. Add the garlic and sauté for one minute. Stir in flour, lemon juice, Italian seasoning, and chicken stock. Return the pan to the heat. Stir until the mixture is smooth and simmer for a few minutes.
Add in the half & half and let it simmer for a couple of minutes.
Add the shrimp and mushroom mixture back to the pan. Cook for 5 minutes, or until the shrimp are cooked through, and the sauce is thickened to your liking. Add the spinach and parm and stir as the spinach wilts. Season with salt & pepper.
Drain the pasta and toss it with the sauce. Serve with additional grated parmesan if you wish.