Place 1/4 cup water and potatoes in a microwave-safe 8-inch square baking dish. Cover with plastic wrap; pierce plastic 3 or 4 times with a fork. Microwave at HIGH 15 minutes or until potatoes are very tender. Remove plastic wrap; cool. Melt butter in a large Dutch oven over medium heat. Add onion, sage, and garlic; cook 7 minutes or until tender, stirring occasionally. Add stock; bring to a simmer. Remove potato skins; discard skins. Place potatoes, stock mixture, beans, milk, salt, and 1/4 teaspoon black pepper in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in lid; process until smooth. Return pureed soup to pan over medium heat; cook 4 to 5 minutes or until thoroughly heated. Divide soup among 4 bowls; top each serving with 1/4 cup apple and 1 tablespoon yogurt. Sprinkle evenly with remaining 1/4 teaspoon pepper.