Curried Sweet Potato Apple Soup

large (1 pound) sweet potatoes

large (8 ounces) tart apple (such as fuji, honeycrisp or Gala)

olive oil

large onion, coarsely chopped (2 cups)

to 3 teaspoons curry powder

(14 to 14-1/2 ounces) vegetable broth (about 1-3/4 Cups)

unsweetened apple juice

 teaspoon salt

container (6 ounces) plain low-fat yogurt

 cup croutons, optional