While the pasta is boiling, prepare your spinach and mushrooms. In a nonstick skillet set over medium heat, add the olive oil, garlic, and crushed red pepper. Cook for about 1 minute or until the garlic is fragrant. Add the mushrooms and cook for about 6 minutes more until the mushrooms are tender. Stir in the spinach and cook for 2 minutes more until the spinach is sufficiently wilted. Remove the pan from heat and add the spaghetti, reserved pasta water, parsley, and parmesan cheese. Toss to coat.