Make cream of celery soup by combining celery, carrots and onions with heavy cream and vegetable broth. Use butter and flour to thicken the soup, and season with salt and black pepper to taste.
To make cream of celery soup, add 1 cup chopped celery, 1 cup chopped carrots and 1 cup vegetable broth to a saucepan, and bring the mixture to a boil. Cook for about four minutes, or until the vegetables are tender, then drain and set aside the cooked vegetables. Melt 4 tablespoons butter in a large saucepan over medium-high heat, add 2 cups diced onions, and saute until the onions are clear. Whisk in 3 tablespoons flour, and cook for two minutes. Add 5 cups vegetable broth, and continue whisking while bring the broth to a boil.
Add 1 cup uncooked, chopped carrots and 1 cup uncooked chopped celery to the broth. Bring the soup to a boil, then reduce the heat, and simmer the soup for 30 minutes. Strain the soup through a fine sieve, and reserve the vegetables. Use a blender to puree the cooked vegetables from the sieve along with 1 1/2 cups of the broth. Stir the pureed vegetable mixture into the pot with the remaining broth, then stir in 1 cup heavy cream and the carrots and celery reserved from the first step of the recipe. Heat the soup to the desired temperature prior to serving.