Wash zucchini and remove skin, if desired. Grate it on the large hole side of the box grater. You should have 2 ½ to 3 cups of shredded zucchini. Set aside.
Cook fettuccini according to instructions on the box. I bring a big pot of water to a boil, salt it, then add pasta and cook until al dente. Reserve ½ cup of pasta water, then drain the pasta.
In a large pan, heat up oil. Add chopped onion and cook for 3 to 4 minutes. Add garlic and saute for 2 to 3 minutes.
Add zucchini to pan and stir in. Season with salt and pepper. Cook for 4 to 5 minutes.
Add 1 teaspoon of lemon zest and 2 teaspoons of lemon juice to the pan. Stir in. Simmer for 2 minutes.
Add cream to zucchini and stir in. Bring to a simmer.
Add Parmesan cheese to the sauce and stir in to melt.
Add pasta water, a little bit at a time, until the sauce reaches desired consistency.
Add cooked pasta to the sauce and toss to coat.
Add more pasta water if needed.
Remove dish from heat. Season with salt and pepper.
Serve garnished with chopped parsley and crushed red pepper flakes.