Preheat oven to 350. Bake sweet potatoes for 1 to 1 1/2 hours or until very soft. Meanwhile add 1 tbsp. of the oil to a medium non stick saucepan and saute the celery and carrots for about 5-7 minutes until they get soft. Stirring occasionally. Add the onions and saute for another 5 minutes or so until the onions are translucent. Add salt and pepper. Stir occasionally.
When the sweet potatoes are done, set aside and let cool. When they are cool enough to handle scoop out the flesh and add to a large bowl and mash. Add the vegetable mixture, eggs, oil, flour, half the parsley and salt and pepper to the sweet potatoes. Stir to combine. Spray a large casserole dish with cooking spray and add mixture. Cover and bake for 30-40 minutes until cooked through. The last 5-7 minutes take off the cover and let the top brown. Sprinkle the remaining parsley on top and serve.