Bring a pot of water to a boil and generously salt it. Once boiling, add the fettuccine and cook al dente. Set aside half a cup of the pasta water before draining.
Prepare the leek by cutting the leafy part and using only the white part of the leek. Save the leaves for another day to flavor your stock or soups. Wash the white part of the leek in running water to take out any soil or grime. Then slice the leek lengthwise, take a piece and slice again lengthwise and cut thinly into quarters.
In a pan, heat the oil in medium heat and sauté the mushrooms for about 2-3 minutes until starting to brown. Add the sliced leeks and sauté for another 2-3 minutes until leeks start to get translucent. Pour the stock and let it cook for a minute or so. Add the cream and stir. Add the dried oregano and season with salt and pepper. Let it simmer for about 10 minutes or until the sauce starts to thicken. Add half the parmesan cheese and lower heat. You can add more stock or cream depending on your preference.
Add the cooked fettuccine into the sauce and stir to coat the pasta with the sauce. You can add a splash of the pasta water if it's too thick. Let it cook for a minute or two and then turn off heat. Add the chopped parsley and top with more parmesan cheese before serving.