Make up your vegetable broth. I usually use 6 cups of boiling water, 1 pkg of onion soup mix and 1 extra large vegetable boullion cube (that does 2 cups of broth, like Knorr.).
In a small saucepan, add 1 cup celery, 1 cup carrots and 1 cup vegetable broth. Bring to a boil and cook for about 4 minutes. Set aside.
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 well until mixture boils.
Add uncooked 1 cup celery and 1 cup carrots. Bring mixture to a boil, reduce heat and simmer covered for about 30 minutes.
When cooked, use a slotted spoon to strain out much of the vegetables, put in blender with a cup or two of the stock and puree (my family likes a bit extra of the vegetables to remain unblended to add to the heartiness, but original recipe said run through a fine sieve and blend all the vegetables.).
Stir puree back into the pot with the liquid. Stir in the reserved vegetables and fat-free evaporated milk (original recipe asked for 1 cup heavy cream.).