Spicy curry powder and tart apples pair perfectly with sweet potatoes in this warm and comforting soup.
Pierce sweet potatoes and apples with fork tines. Microwave sweet potatoes and apple on high until apple is very tender, about 6 to 7 minutes. Remove apple; set aside until cool enough to handle. Continue microwaving sweet potatoes on high until tender, about 4 to 5 minutes longer; set aside until cool enough to handle.
Meanwhile, in large saucepan over medium heat, heat oil; add onion and curry powder; cook and stir until onion begins to brown, about 5 minutes. Add broth; bring to a boil, reduce heat to low; simmer 5 minutes. Transfer broth mixture to bowl of food processor; reserve saucepan. Halve sweet potatoes and apple; remove apple core; scoop potato and apple pulp from skin; add to processor. Add salt; whirl until very smooth, gradually adding apple juice through processor feed tube. Transfer mixture to reserved saucepan; bring to boil over medium heat. Whisk in yogurt; reheat just until hot; do not boil. Serve topped with croutons, if desired.