Cook fettuccine according to the package directions, with 1 Tbsp. olive oil in the water as it cooks (this helps prevent the noodles from sticking together as much during cooking and after draining). When done, drain the pasta, reserving 3/4 cup of the water the pasta was cooked in.
While the pasta cooks, combine the ricotta cheese, lemon juice, lemon rind, salt, black pepper and crushed red pepper in a medium-sized bowl. Set aside.
In a large skillet over medium-high heat, heat the oil, then add the red bell pepper, mushrooms and garlic and sautee for ~3 minutes.
Add the pasta, spinach, ricotta cheese mixture and reserved pasta water to the skillet and cook for ~2 minutes until the spinach wilts and sauce just starts to boil. *Note: If mixture is too thick, add a little hot water to thin it out. If it's a little too thin, add in some extra ricotta cheese to thicken it up.*
Taste test and add additional salt, black pepper and/or crushed red pepper to taste.
Serve hot.