In a hurry? Caramelize the onions on top of the stove. Cook 1 pound chopped onions in 1 tablespoon butter and 1 tablespoon oil in a heavy skillet over medium heat 12 to 15 minutes, stirring often, until tender.
Bake sweet potatoes at 425° for 1 hour, or until very soft. Cool 20 minutes; scoop out the pulp (about 4 cups).
Process sweet potato pulp, half of Oven-Browned Onions, and 4 cups chicken broth in a food processor until mixture is smooth.
Pour sweet potato mixture into a Dutch oven; add remaining chicken broth, salt, pepper, and nutmeg. Bring to a boil; reduce heat to medium, and simmer, stirring occasionally, 20 minutes. Serve soup with remaining Oven-Browned Onions.