Trim any root hairs from 500 grams Japanese sweet potato and wash well. Cut into cubes about ¾ inch (2 centimeters) wide. Add them to a pot and fill with water until the sweet potatoes are covered.
Bring the water to a rolling boil, then reduce the heat to maintain a gentle simmer. Cook the sweet potatoes for 12–14 minutes, or until tender enough for a toothpick to slide through easily.
While the sweet potatoes cook, make the dressing by mixing together ½ cup Greek yogurt and ½ teaspoon salt.
When the sweet potatoes are done, drain them well and add them to the dressing while still hot. Stir to coat; it’s fine if some of the potatoes get lightly mashed.
Add 25 grams walnuts and 20 grams dry-cured ham, then mix until evenly combined.