Melt butter in a small saucepan over medium heat. Whisk in flour until smooth, then add in milk and chicken broth. Heat, stirring until thickened (5-7 min).
Beat the egg yolks in a small bowl. Add a little of the warm custard mixture slowly to the yolks, stirring constantly. Stir the tempered yolks into the saucepan. Heat and stir 1-2 minutes over low heat.
Remove the custard from the heat and stir in pureed carrots, onion, parsley, thyme, salt, and white pepper. Set mixture in the refrigerator to cool to room temperature, stirring often.
Preheat your oven to 350F.
In a large bowl, beat egg whites until frothy. Add cream of tarts and beat until the mixture just barely forms stiff peaks form.
Fold the egg whites into the vegetable mixture, gently, by making S-shaped scooping and pressing motions. (It is ok if some streaks of white remain.)
Spoon mixture into an ungreased 5 c soufflé dish, or into smaller ramekins.
Bake, uncovered on the center rack for 35-45 min for a large dish, or 25-35 min for the smaller dishes.
When the soufflé is done it will be puffy and browned.