Chop spinach coarsely. Combine with chicken broth and carrots in 2-3 quart pot. Cook 5-10 minutes until carrots are tender and spinach has wilted. Remove from heat.
Meanwhile, saute onion and garlic very gently over low to medium heat in butter, about 20 to 30 minutes. onions should be very tender and translucent, but garlic should not be browned! Add flour and cook, stirring constantly 5 to 10 minutes. Combine spinach/broth and onion/garlic mixtures in food processor or blender in small batches. Puree until smooth. Clean pot and return soup to pot.
Add cream, whipping cream and salt and pepper to taste. Heat until hot, but not boiling. Garnish with a dollop of sour cream and garlic croutons.