How to Make It
Heat butter and oil in a large Dutch oven over medium until butter melts. Add leek and chopped apple; cook 6 minutes, stirring occasionally. Stir in curry powder; cook 1 minute. Increase heat to medium-high. Add wine; cook 2 minutes or until liquid is reduced and slightly syrupy, stirring to scrape up browned bits. Add squash and stock to pan; bring to a boil. Reduce heat to medium-low, cover, and simmer 8 minutes or until apple and squash are very tender.
Place half of the squash mixture in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Cover opening in lid with a kitchen towel (to avoid splatters); blend until smooth. Repeat procedure with remaining half of squash mixture. Return soup to pan; stir in maple syrup, vinegar, salt, and pepper. Combine yogurt and 2 tablespoons water in a bowl. Divide soup among 4 bowls; top with yogurt mixture and julienne-cut apple.