In large saucepan, heat oil over medium heat. Add onion and, stirring frequently, cook for 5 minutes or until golden brown.
Peel, core and slice 3 1⁄2 of the apples. Add to pan along with pumpkin, potato, curry powder, ginger and cinnamon. Stir to combine. Add 3 cups water; cover and simmer for 30 minutes or until pumpkin is tender.
Transfer to food processor and process until smooth. Return to pan, add milk and whisk to combine. Cook over low heat until heated through.
Meanwhile, thinly slice remaining 1⁄2 apple (unpeeled). Spoon soup into 4 mugs or soup bowls and top with cashews and apple slices.