Place the carrots, celery and onion in a shallow oven-proof dish to fit.
Dot with the butter and sprinkel with the sugar, ginger and mace.
Pour 2 cups of the broth into the bottom of the dish.
Cover tightly with aluminum foi. Bake for 2 hours.
Remove the vegetables from the oven and transfer them, along with any liquid, toa large, heavy pot. Addd 3 cups of the broth. Bring to a boil, reduce the heat to medium and simmer for 10 minutes, partiallly covered. Cool slightly.
Puree the soup along with the cooked beets in a blender or a food processor until smooth. Return to the pot. season with salt and pepper. Add some of the remaining broth for a thinner soup, if desire.