To make the minted yogurt, combine the yogurt, mint leaves, and 1/4 teaspoon of the salt. Cover and store in the refrigerator until ready to use.
Heat the butter or oil in a large saucepan on medium heat. Add the onion and cook until translucent, about 5 minutes. Add the ginger and garlic and cook until soft, about 2-3 minutes (try not to let the garlic brown!). Stir in the curry powder and stir for about 1 minute, scraping the bottom of the pan. Add apples, cider, water, and salt and bring to a boil. Reduce heat to low and simmer until the apples are completely soft, about 30 minutes.
Puree the bisque in batches and return to saucepan. Season with additional salt to taste. Serve warm or at room temperature topped with a dollop of minted yogurt for each bowl.