In a small saucepan add 1 cup celery, 1 cup carrots, 1 cup vegetable broth. Bring to a boil and cook until tender, about 4 minutes. Drain and reserve cooked vegetables. In a large saucepan, over medium high heat, melt butter. Add onions and saute until clear. Whisking, add flour, cook 2 minutes making sure flour does not brown. Add 5 cups vegetable broth and continue to whisk until mixture boils, making sure to reach the bottom of the pan. Add uncooked 1 cup celery and 1 cup carrots. Bring mixture to a boil, reduce heat, and simmer about 30 minutes. When cooked strain soup through a fine sieve into a clean pot reserving vegetables. In a blender puree vegetables with 1 1/2 cups of liquid. Stir puree into pot with liquid. Stir in cream, reserved vegetables. Heat and serve.